56 Air Brakes Practice Questions & Answers

1. 1. An air brake vehicle combines three separate braking systems. Which set names them correctly?

   • A.Foot brakes, antilock brakes, and the engine exhaust brake
   • B.Drum brakes, disc brakes, and the spring brake chambers
   • C.Service brakes, parking brakes, and emergency brakes✓ Answer
   • D.Service brakes, engine retarder, and trailer hand valve

   The manual describes an air brake vehicle as three braking systems in one: service, parking, and emergency. Retarders and antilock brakes are driving aids layered on top of those systems, and drum versus disc describes the type of foundation brake, not a separate system.

   Source: AAMVA Model CDL Manual, Section 5 (Air Brakes)Report a problem with this question

2. 2. On a tractor with air brakes, what actually provides emergency braking if the air system loses pressure?

   • A.The engine retarder, which slows the drive wheels when the compressor fails
   • B.The service chambers, using air stored in the wet tank for one last stop
   • C.Spring brakes, which air holds off and which apply when pressure is lost✓ Answer
   • D.A hydraulic reserve pump that takes over once the air tanks are empty

   Emergency braking comes from powerful mechanical springs that air pressure holds compressed while you drive. When system pressure falls into the manufacturer's range, the air can no longer hold the springs back and they apply the brakes without any action by the driver.

   Source: AAMVA Model CDL Manual, Section 5 (Air Brakes — spring brakes)Report a problem with this question

3. 3. What job does the air compressor governor do?

   • A.It controls when the compressor pumps air into the tanks and when it stops✓ Answer
   • B.It limits how much pressure the foot valve can send to the brake chambers
   • C.It vents air from the tanks any time pressure climbs above a safe level
   • D.It holds engine speed steady so the compressor turns at a constant rate

   The governor watches tank pressure and decides when the compressor pumps: at cut-out it stops air from being pumped into the tanks, and at cut-in it lets pumping resume. Releasing dangerous overpressure is the separate job of the safety relief valve.

   Source: AAMVA Model CDL Manual, Section 5 (Air Brakes — air compressor governor)Report a problem with this question

4. 4. At about what pressures does an air compressor governor normally cut out and cut back in?

   • A.Cuts out around 125 psi and cuts back in around 100 psi✓ Answer
   • B.Cuts out around 85 psi and cuts back in around 45 psi
   • C.Cuts out around 150 psi and cuts back in around 125 psi
   • D.Cuts out around 100 psi and cuts back in around 60 psi

   The model manual gives roughly 125 psi as the cut-out point and roughly 100 psi as the cut-in point, so the compressor cycles inside a narrow band well above the low air warning. Numbers near 60 psi belong to the warning range, not to governor operation.

   Source: AAMVA Model CDL Manual, Section 5 (Air Brakes — governor cut-in and cut-out)Report a problem with this question

5. 5. The safety relief valve on the supply tank lets air escape. What does that usually mean?

   • A.Pressure has fallen so far that the low air warning is about to sound
   • B.The tanks are full of water and the valve is blowing the moisture out
   • C.The air dryer has finished its cycle and is venting the oil it collected
   • D.Something is wrong with the governor and the compressor keeps pumping✓ Answer

   The safety relief valve is a last-resort protection that opens only when tank pressure exceeds its setting, so air escaping from it means pressure went too high. The usual cause is a governor that failed to cut the compressor out, and the vehicle should be repaired before it is driven.

   Source: AAMVA Model CDL Manual, Section 5 (Air Brakes — safety relief valve)Report a problem with this question

6. 6. Why must air tanks without automatic drains be drained by hand at the end of each working day?

   • A.Air left overnight leaks away and the spring brakes will release on their own
   • B.Empty tanks let the compressor build full pressure faster the next morning
   • C.Draining resets the governor so it cuts out at the right pressure again
   • D.Water and compressor oil collect there and can freeze and block the brakes✓ Answer

   Compressed air carries water vapour and a small amount of compressor oil, and both settle in the bottom of the tanks. In cold weather that liquid can freeze inside valves and lines and stop the brakes from working, which is why manual drain valves are opened daily.

   Source: AAMVA Model CDL Manual, Section 5 (Air Brakes — draining air tanks)Report a problem with this question

7. 7. What is an alcohol evaporator in an air brake system for?

   • A.Raising the boiling point of the fluid in the wheel brake cylinders
   • B.Cutting the risk of ice forming in air lines and valves in cold weather✓ Answer
   • C.Cooling the compressed air so less water condenses inside the tanks
   • D.Burning off the oil the compressor pushes into the tanks as it works

   The evaporator puts a small amount of alcohol into the air system so that any moisture carried along is less likely to freeze. It reduces ice risk but does not remove the water, so the tanks must still be drained.

   Source: AAMVA Model CDL Manual, Section 5 (Air Brakes — alcohol evaporator)Report a problem with this question

8. 8. What is the job of the supply tank, often called the wet tank?

   • A.It feeds air only to the trailer through the emergency line glad hand
   • B.It powers the low air warning buzzer after the engine has been shut off
   • C.It stores the reserve air the spring brakes need to stay released
   • D.It takes the air straight from the compressor so oil and water settle✓ Answer

   The supply tank is the first tank the compressor feeds, and it is where most of the moisture and oil carried in the hot air drop out before the air moves on. Vehicles with an air dryer treat the air on the way in, which is why that tank still needs regular draining.

   Source: AAMVA Model CDL Manual, Section 5 (Air Brakes — air storage tanks and air dryer)Report a problem with this question

9. 9. What happens to the air in the tanks each time the driver presses and releases the brake pedal?

   • A.The same air flows back into the tanks, so pressure hardly changes at all
   • B.Some air is used and let out to the air, so tank pressure drops a little✓ Answer
   • C.Air only shifts between the front and rear tanks, so the total stays even
   • D.The compressor refills it at once, so the gauge cannot register any drop

   The treadle valve sends tank air out to the brake chambers, and on release that air is exhausted to the atmosphere rather than returned. Because every application spends air, repeated or fanned braking can pull pressure down faster than the compressor replaces it.

   Source: AAMVA Model CDL Manual, Section 5 (Air Brakes — foot brake valve)Report a problem with this question

10. 10. What does the application pressure gauge tell the driver?

    • A.How much air is left in the service air tanks at that moment
    • B.How much the brake linings have worn since the last adjustment
    • C.How much air pressure is being applied to the brakes at that moment✓ Answer
    • D.The pressure at which the governor will shut the compressor off

    This gauge reads the pressure being delivered to the brakes at the moment you press the pedal, which is different from the supply gauge that reads what is stored in the tanks. A rising application pressure needed to hold the same speed is an early sign of fading or badly adjusted brakes.

    Source: AAMVA Model CDL Manual, Section 5 (Air Brakes — application pressure gauge)Report a problem with this question

11. 11. What is a wig wag on an older air brake vehicle?

    • A.A valve that drains water out of the bottom of the tanks while driving
    • B.A lever that applies the trailer brakes without using the foot pedal
    • C.A mechanical arm that drops into the driver's view when air gets low✓ Answer
    • D.A gauge marking that shows how far the push rod travels when braking

    The wig wag is a mechanical form of the low air warning: an arm swings down where the driver cannot miss it when pressure gets low. The automatic type rises back out of view once pressure climbs above roughly 55 psi, and the manual type must be placed back by hand and will not stay up until pressure recovers.

    Source: AAMVA Model CDL Manual, Section 5 (Air Brakes — wig wag warning device)Report a problem with this question

12. 12. In an s-cam drum brake, what forces the brake shoes out against the drum?

    • A.The return spring pulls the shoes out as air leaves the chamber
    • B.Fluid from a master cylinder pushes a piston into each brake shoe
    • C.Air in the brake chamber moves the push rod, which twists the s-cam✓ Answer
    • D.Air enters the drum itself and expands, pushing the shoes outward

    Air pressure acts on a diaphragm in the brake chamber, pushing the push rod out; the slack adjuster converts that push into rotation of the s-cam, which spreads the shoes. The return spring does the opposite job, pulling the shoes back when the air is released.

    Source: AAMVA Model CDL Manual, Section 5 (Air Brakes — s-cam brakes)Report a problem with this question

13. 13. Why does too much push rod travel in an s-cam brake matter?

    • A.The brake chamber loses its seal and leaks air into the drum housing
    • B.The spring brakes will drag on every time the vehicle slows down
    • C.The brakes may not apply fully, above all when the drums run hot✓ Answer
    • D.The compressor will run non-stop and overheat the air in the tank

    Long stroke means the chamber runs out of usable travel before the shoes are pressed hard against the drum, so braking force falls off. Heat makes it worse because a hot drum expands away from the shoes, which is exactly when a long downgrade needs full braking.

    Source: AAMVA Model CDL Manual, Section 5 (Air Brakes — slack adjusters and push rod stroke)Report a problem with this question

14. 14. On an air-operated disc brake, what does the job the s-cam does in a drum brake?

    • A.A wheel cylinder that pushes brake fluid to both sides of the caliper
    • B.A power screw that the push rod turns to clamp the pads on the rotor✓ Answer
    • C.A slack adjuster that draws the pads inward when the driver brakes
    • D.A wedge that spreads two pistons apart inside the brake drum housing

    Air disc brakes use the same air chamber and push rod, but the rod turns a power screw instead of an s-cam, and the screw squeezes the caliper pads onto the rotor. The system is still all air; no hydraulic fluid is involved anywhere in it.

    Source: AAMVA Model CDL Manual, Section 5 (Air Brakes — disc brakes)Report a problem with this question

15. 15. What does the service line between tractor and trailer carry?

    • A.A steady supply of air that keeps the trailer spring brakes released
    • B.Electric signals that run the trailer's brake lights and marker lamps
    • C.Air that flows only after the tractor protection valve has closed
    • D.Air controlled by the foot pedal or hand valve to brake the trailer✓ Answer

    The service line, also called the control or signal line, carries the pressure the driver commands with the foot valve or the trailer hand valve, and that pressure applies the trailer service brakes. Keeping the trailer charged is the separate job of the emergency line.

    Source: AAMVA Model CDL Manual, Section 6 (Combination Vehicles — service air line)Report a problem with this question

16. 16. What does the emergency (supply) air line do?

    • A.It sends the driver's pedal pressure to the trailer service chambers
    • B.It lets the trailer draw extra air once the low air warning sounds
    • C.It carries air back from the trailer so the dash gauge can read it
    • D.It keeps the trailer's air tanks charged and controls its emergency brakes✓ Answer

    The emergency line keeps the trailer air tanks supplied and, by holding pressure in that line, keeps the trailer emergency brakes from applying. If the line loses pressure, the trailer brakes come on, which is what protects a trailer that breaks away.

    Source: AAMVA Model CDL Manual, Section 6 (Combination Vehicles — emergency air line)Report a problem with this question

17. 17. What does the tractor protection valve do if the trailer breaks away or develops a big leak?

    • A.It opens both lines wide so the trailer gets every bit of air that is left
    • B.It shuts the lines and saves the air the tractor needs for its own brakes✓ Answer
    • C.It stops the compressor so the leak cannot empty the supply tank as well
    • D.It sets off the alarm and locks both dash control knobs in the in position

    The valve isolates the trailer lines so a runaway leak cannot drain the tractor's own air supply, leaving the power unit able to stop itself. It closes automatically when air pressure falls into the manufacturer's range of roughly 20 to 45 psi, and the driver can also close it with the dash control.

    Source: AAMVA Model CDL Manual, Section 6 (Combination Vehicles — tractor protection valve)Report a problem with this question

18. 18. What does the red, eight-sided knob on the dash control?

    • A.The parking brakes on the tractor, leaving the trailer brakes released
    • B.The supply of air to the trailer, and the tractor protection valve with it✓ Answer
    • C.The trailer service brakes, used to test them while the vehicle is still
    • D.The pressure the foot valve may send to the front steering axle brakes

    The red eight-sided knob is the trailer air supply control: pushed in it opens the tractor protection valve and charges the trailer, pulled out it closes the valve and sets the trailer emergency brakes. It is the control a driver uses before disconnecting from a trailer.

    Source: AAMVA Model CDL Manual, Section 6 (Combination Vehicles — trailer air supply control)Report a problem with this question

19. 19. What is the yellow, diamond-shaped knob in the cab used for?

    • A.Putting on and releasing the parking brakes on the tractor✓ Answer
    • B.Cutting the air off to a trailer that is being left behind
    • C.Applying the trailer brakes by themselves for a road test
    • D.Draining the air out of the tanks at the end of the day

    The yellow diamond-shaped knob is the parking brake control for the power unit: pulling it out applies the tractor spring brakes and pushing it in releases them. Shutting off trailer air is the red eight-sided knob, and the hand valve is only for testing trailer brakes.

    Source: AAMVA Model CDL Manual, Section 6 (Combination Vehicles — parking control valve)Report a problem with this question

20. 20. Within what range of air pressure will the spring brakes on a typical tractor come fully on by themselves?

    • A.Somewhere between 20 and 45 psi✓ Answer
    • B.Somewhere between 55 and 75 psi
    • C.Somewhere between 5 and 15 psi
    • D.Somewhere between 80 and 100 psi

    The manual gives roughly 20 to 45 psi, with many vehicles acting near the low end of that band, as the pressure at which air can no longer hold the springs back. Because that point is well below the 55 psi warning, a driver should stop safely as soon as the warning appears rather than wait for the springs to grab.

    Source: AAMVA Model CDL Manual, Section 5 (Air Brakes — spring brakes)Report a problem with this question

21. 21. What is a modulating control valve used for?

    • A.Cutting front brake power when the road surface turns slippery
    • B.Holding the trailer brakes on while the rig is parked on a grade
    • C.Applying the spring brakes gradually if the service brakes fail✓ Answer
    • D.Adjusting how much air the governor sends into the wet tank

    A modulating control valve lets the driver put the spring brakes on by degrees with a control lever, so a vehicle whose service brakes have failed can be slowed under some control instead of locking up. The further the lever moves, the harder the spring brakes apply.

    Source: AAMVA Model CDL Manual, Section 5 (Air Brakes — modulating control valve)Report a problem with this question

22. 22. What does an antilock braking system do for the driver of an air brake vehicle?

    • A.It keeps the wheels from locking so the driver can still steer and brake✓ Answer
    • B.It cuts the distance needed to stop on dry pavement by roughly a third
    • C.It brakes for the driver whenever the vehicle closes on traffic ahead
    • D.It pumps the brake pedal for the driver so every stop is a shorter one

    ABS is an add-on to the normal braking system that prevents wheel lockup so the driver keeps steering control during hard braking; it does not shorten stopping distance. Because the system does the modulating, the driver brakes normally and must not pump the pedal.

    Source: AAMVA Model CDL Manual, Section 5 (Air Brakes — antilock braking systems)Report a problem with this question

23. 23. Where is the antilock brake malfunction lamp on a trailer, and what colour is it?

    • A.A green lamp mounted on the tractor's instrument panel
    • B.A red lamp mounted on the right side of the trailer
    • C.A yellow lamp mounted on the rear of the trailer body
    • D.A yellow lamp mounted on the left side of the trailer✓ Answer

    Trailers and converter dollies required to have ABS carry a yellow malfunction lamp on the left side so the driver can see it in the mirror. If that lamp stays on, the antilock function may be out, but the vehicle still has its ordinary air brakes.

    Source: 49 CFR 393.55; AAMVA Model CDL Manual, Section 5 (Air Brakes — ABS malfunction lamp)Report a problem with this question

24. 24. How is the air compressor on a truck driven?

    • A.By exhaust gas routed from the turbocharger into the pump housing
    • B.By the turning drive axle, so it only pumps while the truck moves
    • C.By the engine, through gears or a belt, so it runs with the engine✓ Answer
    • D.By its own electric motor that the driver switches on before a trip

    The compressor is driven off the engine by gears or a v-belt, so it turns whenever the engine is running and the governor decides when that turning actually pumps air. That is also why pressure can be rebuilt while standing still at a fast idle.

    Source: AAMVA Model CDL Manual, Section 5 (Air Brakes — air compressor)Report a problem with this question

25. 25. A truck has a dual air brake system. What does that mean?

    • A.One system runs the brakes and the other runs the horn and the wipers
    • B.Each has its own tanks and lines, and one still works if the other fails✓ Answer
    • C.One takes over from the other as soon as pressure passes 60 psi
    • D.One system serves the tractor and the other serves the trailer brakes

    A dual system is two complete air systems, primary and secondary, sharing one compressor and set of controls but with separate tanks and lines. If one side loses pressure the other still stops the vehicle, though stopping distance grows, so the warning must be treated as an emergency.

    Source: AAMVA Model CDL Manual, Section 5 (Air Brakes — dual air brake systems)Report a problem with this question

26. 26. Before you begin the air brake check, how should the parked vehicle be set up?

    • A.With the engine held at a fast idle, so the tanks stay at governor cut-out pressure.
    • B.On level ground with the wheels chocked, so it cannot roll when the parking brake is released.✓ Answer
    • C.With the parking brake left applied throughout, so that no wheel chocks are needed at all.
    • D.On a slight downgrade with a gear engaged, so the engine helps hold the vehicle still.

    The leakage and warning tests are run with the engine off and the parking brake released, so nothing but chocks on level ground is holding the vehicle. Setting up any other way either makes the test impossible or lets the vehicle roll away.

    Source: AAMVA Model CDL Manual, Air Brakes section (air brake system check)Report a problem with this question

27. 27. With the engine off and the parking brake released, you fan the brake pedal to bleed pressure down. What must the low air pressure warning do?

    • A.The warning should stay off until the governor cuts the compressor back in.
    • B.The warning must come on before tank pressure drops below 55 psi.✓ Answer
    • C.The warning may wait until about 20 psi, when the spring brakes set themselves.
    • D.The warning is required only on vehicles that have no spring parking brakes.

    Federal equipment rules require a warning signal that operates before air pressure falls below 55 psi, which is well above the 20 to 45 psi band where the spring brakes apply, so the driver gets time to stop while the service brakes still work.

    Source: 49 CFR 393.51; AAMVA Model CDL Manual, Air Brakes sectionReport a problem with this question

28. 28. You keep fanning off air after the warning comes on. At what pressure should the parking brake knob and the tractor protection valve close by themselves?

    • A.Between 120 and 140 psi, the band in which the governor cuts out.
    • B.Between 20 and 45 psi, the range in which the spring brakes apply.✓ Answer
    • C.Between 85 and 100 psi, the minimum pressure needed to drive.
    • D.Between 55 and 75 psi, just after the low air warning has come on.

    Spring brakes are held off by air, so when supply pressure falls into the 20 to 45 psi range the springs take over and the valves pop out automatically. The warning comes much earlier, which is why it is a warning and not the brake application itself.

    Source: AAMVA Model CDL Manual, Air Brakes section (air brake system check)Report a problem with this question

29. 29. The gauge shows the tanks filling far more slowly than the manual allows. What should you do?

    • A.Raise the idle and set out, because pressure builds faster at highway speed.
    • B.Keep the vehicle parked and have the compressor and governor repaired first.✓ Answer
    • C.Drive the first few miles gently so the compressor can catch up with demand.
    • D.Drain every tank fully, since trapped water is the only cause of slow buildup.

    Slow buildup means the compressor cannot replace the air that normal braking uses, so a hard stop or a downgrade can drain the system to the point where the spring brakes apply. A defect found in the inspection has to be corrected before the vehicle is driven.

    Source: AAMVA Model CDL Manual, Air Brakes section; 49 CFR 396.3Report a problem with this question

30. 30. How should you check the air compressor governor, and what readings are correct?

    • A.Build pressure until the safety valve pops near 150 psi, then fan down until the low air warning light appears.
    • B.Build pressure until the compressor cuts out near 120 to 140 psi, then fan down until it cuts in near 100 psi.✓ Answer
    • C.Build pressure until the spring brakes release near 45 psi, then fan down until the control knob pops out again.
    • D.Build pressure until the compressor cuts out near 60 to 75 psi, then fan down until it starts pumping again at 55 psi.

    The governor stops the compressor from pumping at cut-out, about 120 to 140 psi, and starts it again at cut-in, about 100 psi. A cut-out far below that band means the system will run short of air under heavy braking.

    Source: AAMVA Model CDL Manual, Air Brakes section (governor cut-in and cut-out)Report a problem with this question

31. 31. Engine off, system fully charged, brakes released. What is the greatest air loss allowed in one minute?

    • A.Under 3 psi for a single vehicle and under 4 psi for a combination.
    • B.Under 5 psi for a single vehicle and under 7 psi for a combination.
    • C.Under 2 psi for a single vehicle and under 3 psi for a combination.✓ Answer
    • D.Under 10 psi for a single vehicle and under 15 psi for a combination.

    With the brakes released only the supply side is pressurized, so the limits are tighter than with the pedal applied: under 2 psi per minute single and under 3 psi combination. The 3 and 4 psi figures belong to the applied test.

    Source: AAMVA Model CDL Manual, Air Brakes section (static leakage test)Report a problem with this question

32. 32. How is the applied leakage test done on a tractor-trailer, and how much loss is allowed?

    • A.Pump the pedal repeatedly from full pressure; loss must stay under 6 psi in two minutes.
    • B.Hold the pedal down with at least 90 psi in the tanks; loss must stay under 4 psi in a minute.✓ Answer
    • C.Hold the pedal down with about 50 psi in the tanks; loss must stay under 2 psi in a minute.
    • D.Hold the trailer hand valve applied alone; loss must stay under 3 psi in three minutes.

    Applying the pedal pressurizes the service side as well, so the test must start from a nearly full system, around 90 psi or more, and a combination may lose no more than 4 psi in one minute. Pumping the pedal would make the reading meaningless.

    Source: AAMVA Model CDL Manual, Air Brakes and Combination Vehicles sections (applied leakage test)Report a problem with this question

33. 33. How much air pressure must a dual air brake system have before you release the parking brake and drive?

    • A.At least 100 psi in both the primary and the secondary system.✓ Answer
    • B.At least 150 psi in the supply tank that feeds all the other tanks.
    • C.At least 45 psi in either system, enough to release the springs.
    • D.At least 55 psi in the primary system, whatever the secondary shows.

    Each half of a dual system brakes different axles, so both must be up to normal operating pressure before the vehicle moves; 100 psi is the figure the model manual gives. Fifty-five psi is only the warning threshold, not a pressure fit for driving.

    Source: AAMVA Model CDL Manual, Air Brakes section (dual air brake systems)Report a problem with this question

34. 34. What must you do before pulling on a slack adjuster to check brake adjustment?

    • A.Park on level ground, leave the parking brakes set, and jack the axle clear.
    • B.Park on level ground, apply the service brake, and have a helper hold the pedal.
    • C.Park on level ground, drain the tanks dry, and pull with the engine running.
    • D.Park on level ground, chock the wheels, release the parking brakes, wear gloves.✓ Answer

    The slack adjuster can only be moved when the brakes are released, and releasing them removes the only thing holding the vehicle, so chocks come first; gloves protect against sharp and hot parts. Leaving the parking brake set makes the check impossible.

    Source: AAMVA Model CDL Manual, Air Brakes section (checking brake adjustment)Report a problem with this question

35. 35. You pull hard on a slack adjuster and the push rod moves about an inch and a half. What does that tell you?

    • A.The brakes are correctly set, since an inch and a half is the target stroke.
    • B.The brakes are merely cold, so the free play closes up as they warm up.
    • C.The brakes are out of adjustment and must be fixed before you drive.✓ Answer
    • D.The brakes are worn out, so the linings have to be replaced, not adjusted.

    More than about one inch of movement where the push rod attaches means the stroke is too long, so the chamber runs out of travel before full braking force is reached. Out-of-adjustment brakes also overheat and fade first on a grade.

    Source: AAMVA Model CDL Manual, Air Brakes section (checking brake adjustment)Report a problem with this question

36. 36. An automatic slack adjuster is found beyond the legal pushrod stroke limit. What is the right response?

    • A.Back the adjuster off a few turns, since over-adjustment is what lengthens stroke.
    • B.Drive on to the shop as it is, since the adjuster corrects itself after some braking.
    • C.Adjust it by hand at the roadside, since automatic adjusters are made to be reset.
    • D.Have a qualified mechanic find and repair the fault that the long stroke reveals.✓ Answer

    An automatic adjuster that lets stroke run long is itself failing, so adjusting it by hand only masks the defect and it drifts out again in a few stops. Federal equipment rules require the brake to be repaired, not manually reset.

    Source: 49 CFR 393.53; AAMVA Model CDL Manual, Air Brakes sectionReport a problem with this question

37. 37. How large a crack in a brake drum or disc is still acceptable?

    • A.A crack no longer than one half the width of the friction area.✓ Answer
    • B.A crack no longer than twice the width of the whole friction area.
    • C.Any crack at all, of any length, puts the drum out of service.
    • D.A crack no longer than one third of the length of the whole drum.

    The model manual sets the limit at one half the width of the friction area; a longer crack can let the drum break apart under the heat and force of heavy braking. Small cracks within that limit are common and are not a defect.

    Source: AAMVA Model CDL Manual, Air Brakes section (brake drums and discs)Report a problem with this question

38. 38. Why must air tanks without automatic drain valves be drained at the end of each working day?

    • A.Emptying the tanks resets the governor so the compressor cuts in next morning.
    • B.Open drains let the alcohol evaporator refill the lines with fresh vapor.
    • C.Draining relieves tank pressure so the springs are not held compressed all night.
    • D.Water and compressor oil build up in the tanks and can freeze the brakes shut.✓ Answer

    Compressed air carries water vapor and a little compressor oil, and both settle in the tanks; in cold weather that water freezes in valves and lines and the brakes stop working. Daily draining is why tanks have drain valves at the lowest point.

    Source: AAMVA Model CDL Manual, Air Brakes section (air storage tanks and drains)Report a problem with this question

39. 39. How do you test that the parking brake will actually hold the vehicle?

    • A.With the parking brake set, rev the engine in neutral and watch the air gauge.
    • B.With the wheels chocked, apply the parking brake and listen for escaping air.
    • C.With the parking brake set, pull gently forward in a low gear and feel it hold.✓ Answer
    • D.With the service brake held, release the parking brake and let the truck roll.

    A parking brake is only proven by load: with it set, ease forward in a low gear and the vehicle should not move. Listening or watching a gauge tells you nothing about whether the spring brakes can hold the weight of the vehicle.

    Source: AAMVA Model CDL Manual, Air Brakes section (parking brake check)Report a problem with this question

40. 40. At about five miles per hour you apply the service brakes and the vehicle pulls hard to one side. What does that mean?

    • A.The linings are still seating in, so the pull will fade after a few more stops.
    • B.The cargo has shifted to that side, so the load needs restacking and securing.
    • C.Tank pressure is uneven between the two systems, so the governor needs resetting.
    • D.One side is braking harder than the other, so the brakes need service now.✓ Answer

    A pull under braking is uneven braking force across the axle, from an out-of-adjustment brake, a grabbing lining or a leaking chamber. At highway speed the same imbalance can pull the vehicle out of its lane or start a jackknife.

    Source: AAMVA Model CDL Manual, Air Brakes section (service brake check)Report a problem with this question

41. 41. What is the correct way to make a normal stop with air brakes?

    • A.Press the pedal lightly and finish the stop with the trailer hand valve.
    • B.Push the pedal down steadily and control pressure for a smooth stop.✓ Answer
    • C.Press the pedal hard at first and then ease off steadily as you slow.
    • D.Pump the pedal in short bursts so the compressor keeps the tanks full.

    Air brake force follows pedal pressure, so a steady, metered application gives a smooth predictable stop and keeps the load from shifting. Pumping wastes air faster than the compressor can replace it.

    Source: AAMVA Model CDL Manual, Air Brakes section (normal stops)Report a problem with this question

42. 42. What does controlled braking mean when you must stop quickly?

    • A.Apply the brakes as hard as you can without locking the wheels, steering little.✓ Answer
    • B.Apply the brakes gently and steer sharply around whatever blocks your lane.
    • C.Apply the brakes fully until the wheels lock, letting the tires scrub off the speed.
    • D.Apply the parking brake along with the service brake for the shortest stop.

    Locked wheels cannot steer and a locked trailer axle starts a jackknife, so controlled braking keeps the tires just short of lockup where they still hold the road. Large steering inputs during hard braking are what break that grip.

    Source: AAMVA Model CDL Manual, Air Brakes section (emergency stops)Report a problem with this question

43. 43. When using stab braking, when do you apply the brakes again?

    • A.When the wheels have begun rolling again, about a second after release.✓ Answer
    • B.While the wheels are still locked, so the rig keeps scrubbing off its speed.
    • C.When the vehicle has slowed about five miles per hour below your safe speed.
    • D.When the air gauge has climbed back up to the governor cut-out reading.

    Stab braking is full application, release the moment the wheels lock, then reapply once they are rolling, which takes roughly a second. Reapplying while the wheels are still locked leaves the vehicle sliding with no steering.

    Source: AAMVA Model CDL Manual, Air Brakes section (stab braking)Report a problem with this question

44. 44. What is the most important thing to do before starting down a long, steep grade?

    • A.Choose a safe speed and shift into a low gear before the descent begins.✓ Answer
    • B.Set the trailer hand valve so the trailer leads the rig down the grade.
    • C.Switch off the engine brake so the drive wheels keep all of their traction.
    • D.Build the tanks to cut-out and coast in neutral to save fuel on the way down.

    Engine braking in a low gear does most of the work on a grade and the service brakes are only a supplement; once the vehicle is rolling too fast it may be impossible to downshift at all. Coasting in neutral removes the engine braking entirely.

    Source: AAMVA Model CDL Manual, Air Brakes section (braking on downgrades)Report a problem with this question

45. 45. Using proper downgrade braking technique, when should you release the brakes?

    • A.Once the low air warning lights up and the tanks need time to refill.
    • B.Once the vehicle is down to walking pace and the grade begins to flatten.
    • C.Once the pedal starts to feel soft, which shows the linings are heating up.
    • D.Once the rig is about 5 mph below the safe speed, roughly three seconds in.✓ Answer

    Snub braking applies the brakes firmly until speed is about 5 mph below the safe speed, a burst of roughly three seconds, then releases so the linings cool before the next snub. Light continuous pressure keeps heat in the drums and brings on fade.

    Source: AAMVA Model CDL Manual, Air Brakes section (proper braking technique on downgrades)Report a problem with this question

46. 46. What physically happens when drum brakes fade on a long downgrade?

    • A.The drums expand away from the shoes and hot linings grip with less friction.✓ Answer
    • B.The slack adjusters shrink with heat and pull the shoes back off the drums.
    • C.The chamber springs weaken with heat and push the shoes away from the drums.
    • D.The air lines soften with heat and let pressure bleed out of the chambers.

    Heat does two things at once: it expands the drum outward away from the shoes and it chemically changes the lining so it grips less. Both effects mean the same pedal pressure produces less braking, which is why brakes out of adjustment fade first.

    Source: AAMVA Model CDL Manual, Air Brakes section (brake fading or failure)Report a problem with this question

47. 47. You have driven through deep water in below-freezing weather. What should you do about the brakes?

    • A.Drain the air tanks at the roadside so water cannot reach the brake chambers.
    • B.Stop at once and let the brakes air dry before the vehicle is moved again.
    • C.Drive slowly while applying light pressure so friction dries out the linings.✓ Answer
    • D.Pump the pedal hard several times while stopped to force the water back out.

    Wet linings grip poorly and in freezing weather the water can turn to ice in the drums, so the cure is gentle pressure while moving slowly, which heats the friction surfaces and drives the moisture off. Parking wet brakes in the cold can freeze them solid.

    Source: AAMVA Model CDL Manual, Air Brakes section (wet brakes)Report a problem with this question

48. 48. With air brakes, total stopping distance is the sum of which four parts?

    • A.Perception distance, reaction distance, brake lag distance and braking distance.✓ Answer
    • B.Reaction distance, brake lag distance, braking distance and following distance.
    • C.Perception distance, brake lag distance, braking distance and pumping distance.
    • D.Perception distance, reaction distance, braking distance and coasting distance.

    Air takes time to travel from the pedal to each brake chamber, so air-braked vehicles add a brake lag segment that hydraulic vehicles do not have, on top of perception, reaction and braking distance. At highway speed that lag alone adds roughly the length of a tractor-trailer.

    Source: AAMVA Model CDL Manual, Air Brakes section (braking distance with air brakes)Report a problem with this question

49. 49. The low air pressure warning comes on while you are driving. What should you do?

    • A.Shift down and slow up, since the warning clears once road speed is lower.
    • B.Pull the parking brake knob now so the spring brakes set while rolling.
    • C.Keep going to the nearest shop while pumping the pedal to rebuild pressure.
    • D.Stop safely and park as soon as you can, before the spring brakes come on.✓ Answer

    The warning means air is being lost faster than it is replaced, and when pressure reaches the 20 to 45 psi band the spring brakes apply on their own, wherever the vehicle happens to be. Pumping the pedal only empties the tanks faster.

    Source: AAMVA Model CDL Manual, Air Brakes section (low air pressure warning)Report a problem with this question

50. 50. Why must the trailer hand valve never be used to park or to hold speed on a downgrade?

    • A.It applies the tractor brakes only, so the trailer would have no braking at all.
    • B.It sends air to the spring brakes, and holding it overheats the parking chambers.
    • C.Air can leak off and release the brakes, and steady use can start a trailer skid.✓ Answer
    • D.It closes the tractor protection valve, so the tractor would lose its air supply.

    The hand valve works the trailer service brakes, which are held on by air pressure that can leak away and let a parked rig roll. On a grade it brakes the trailer wheels alone, and trailer wheels braked by themselves are what start a trailer skid.

    Source: AAMVA Model CDL Manual, Combination Vehicles section (trailer hand valve)Report a problem with this question

51. 51. What happens if you repeatedly fan the brakes on a long downgrade?

    • A.The governor stays cut in, so the tanks refill faster than in steady braking.
    • B.Air goes out faster than the compressor replaces it, so pressure falls low.✓ Answer
    • C.The spring brakes partly apply each time, adding braking force on the grade.
    • D.The linings cool between applications, which is how fanning prevents fade.

    Every application dumps air to atmosphere when the pedal is released, and a compressor sized for normal use cannot keep up, so tank pressure walks down until the warning sounds and the spring brakes apply. Proper snub braking uses far fewer, firmer applications.

    Source: AAMVA Model CDL Manual, Air Brakes section (braking on downgrades)Report a problem with this question

52. 52. During a pre-trip check on a vehicle with a DUAL air brake system, you run the engine at operating rpm and time the gauges. Which reading meets the air pressure buildup standard for that dual system?

    • A.Pressure climbs from 85 to 100 psi within 45 seconds at operating rpm✓ Answer
    • B.Pressure climbs from 50 to 90 psi within 45 seconds while at governed idle
    • C.Pressure climbs from 50 to 90 psi within 3 minutes at operating rpm
    • D.Pressure climbs from 85 to 100 psi within 3 minutes at governed engine idle

    For a dual air brake system the check is that air pressure builds from 85 to 100 psi within 45 seconds with the engine at operating rpm. Slower buildup than that means the compressor, governor or a leak cannot keep up with normal air use, so the vehicle should not be driven until it is repaired. The 50 to 90 psi in three minutes figure is the separate standard for an older single air brake system, and idle speed is not the condition either check is run at.

    Source: AAMVA Model Commercial Driver License Manual, Air Brakes section (air pressure buildup check)Report a problem with this question

53. 53. An older tractor has a front brake limiting valve with "normal" and "slippery" positions. What does putting the control in "slippery" actually do?

    • A.It cuts air to the front brakes and lengthens the vehicle's stopping distance✓ Answer
    • B.It boosts air to the front brakes so that the steer axle does more of the braking
    • C.It sends air to the front brakes later, timed to arrive after the rear brakes
    • D.It locks out the rear brakes so the steer tires keep their grip on wet pavement

    The "slippery" position reduces the air pressure delivered to the front service brakes by about half, so the vehicle takes longer to stop. Research showed that front wheel braking is effective in all conditions, including wet and icy roads, and does not cause a loss of steering control, so the control should be left in "normal." The valve never adds braking at the front and never disables the rear brakes.

    Source: AAMVA Model Commercial Driver License Manual, Air Brakes section (front brake limiting valve)Report a problem with this question

54. 54. Air brakes do not take hold the instant the pedal goes down, because the air must travel through the lines to each brake chamber. At 55 mph, about how far does the vehicle travel during that brake lag alone?

    • A.About 216 feet, which is really the full braking distance by itself at that speed
    • B.About 61 feet, the same as the average driver's reaction distance at that speed
    • C.About 12 feet, since the delay only affects the trailer brakes behind you
    • D.About 32 feet, which is added on top of perception, reaction and braking distance✓ Answer

    Air takes roughly half a second to move through the lines and apply the brakes, and at 55 mph the vehicle covers about 32 feet in that time. This is why an air brake vehicle needs a fourth element in its total stopping distance that a hydraulic system does not have, and why following distance must be judged with the lag included. The 61 foot and 216 foot figures are the reaction and braking portions at 55 mph, not the lag.

    Source: AAMVA Model Commercial Driver License Manual, Air Brakes section (brake lag and total stopping distance)Report a problem with this question

55. 55. You reach the bottom of a long, steep grade and pull into a rest area. The brakes are still very hot from the descent. What should you do about the parking brake?

    • A.Apply the parking brake now; clamping hot drums helps them cool
    • B.Use the trailer hand valve instead so the tractor brakes cool
    • C.Chock the wheels and leave the parking brake off until they cool✓ Answer
    • D.Set the parking brake and pour water on the drums to cool them

    Holding very hot brakes clamped shut can damage the drums and linings, so the manual says not to use the parking brake right after a long descent; let the brakes cool first and chock the wheels to hold the vehicle. The same caution applies when the brakes are wet in freezing weather, because a set parking brake can freeze the shoes to the drums. The trailer hand valve is never a substitute for the parking brake, and dousing hot drums with water can crack them.

    Source: AAMVA Model Commercial Driver License Manual, Air Brakes section (parking brake use)Report a problem with this question

56. 56. A driver hooks up the tractor-trailer air lines with the service and emergency glad hands crossed. What is the result?

    • A.The trailer brakes still work, but only after a long pedal delay
    • B.The trailer brakes drag because supply air reaches the chambers
    • C.The tractor protection valve closes and no air pressure builds up
    • D.The trailer tanks never charge, so the trailer has no working brakes✓ Answer

    Crossed connections send the supply air into the service line instead of into the emergency line, so the trailer air tanks never fill and there is no stored air to apply the trailer brakes when the driver needs them. The driver can pull away feeling normal, then discover at the first hard stop that only the tractor is braking. Checking is simple: charge the system, shut the engine off, and listen at the glad hands and the trailer for air moving the way it should.

    Source: AAMVA Model Commercial Driver License Manual, Combination Vehicles section (glad hands and air line connections); 49 CFR 393.45Report a problem with this question

Practice questions based on the AAMVA/FMCSA model CDL manual. Requirements vary by state — study your state's official CDL manual before your exam. Official CDL info →