48 Air Brakes Practice Questions & Answers
Every Air Brakes practice question from the CDL Practice Test, with the correct answer and a short explanation.
Start practice test →1. At approximately what air pressure should the low air pressure warning signal come on in a vehicle with an air brake system?
- A.Below 20 psi
- B.Below 100 psi
- C.Below 85 psi
- D.Below 60 psi✓ Answer
The low air pressure warning signal must come on before pressure drops below 60 psi in the air tank (or tank with lowest pressure in a dual system).
2. What causes the air compressor governor to stop the compressor from pumping air (cut-out)?
- A.The parking brake being released
- B.Tank pressure dropping below 60 psi
- C.The engine reaching operating temperature
- D.Tank pressure reaching the 'cut-out' level, about 120 psi✓ Answer
The governor stops the compressor from pumping air when tank pressure reaches the cut-out level (around 120 psi).
3. Why must you drain water from air tanks?
- A.Water and compressor oil can reduce braking, and water can freeze and cause brake failure in cold weather✓ Answer
- B.Draining is only needed once a year
- C.Water increases air pressure to dangerous levels
- D.Water improves braking so it should be kept in the tanks
Compressed air contains water and compressor oil that collect in tanks; if not drained, water can freeze in cold weather and cause brake failure.
4. What is the primary purpose of the safety valve installed in the first (supply) air tank?
- A.To drain water from the tanks
- B.To apply the parking brakes automatically
- C.To warn the driver of low air pressure
- D.To protect the tank and system from too much pressure by releasing air, usually set to open at 150 psi✓ Answer
The safety valve protects the tank and the rest of the system from too much pressure and is usually set to open at 150 psi.
5. What causes brake fade in an air brake system?
- A.Draining water from the tanks
- B.Excessive heat from using the brakes too much, reducing the friction between drums and linings✓ Answer
- C.Too much air pressure in the tanks
- D.Releasing the parking brake too quickly
Brake fade results from excessive heat that reduces the friction between the brake drums and linings, so the brakes lose effectiveness.
6. How do spring (parking) brakes on most large trucks and buses hold the vehicle in place?
- A.Air pressure pushes the springs to apply the brakes
- B.Powerful springs apply the brakes when air pressure is released from the brake chambers✓ Answer
- C.An electric motor clamps the brake drums
- D.Hydraulic fluid locks the wheels
Spring brakes are held back by air pressure; when air is released, powerful springs apply the brakes, which is why they hold even without air.
7. At what air pressure will the spring (parking) brakes typically come on automatically on a tractor?
- A.At exactly 60 psi
- B.Only when pressure reaches 0 psi
- C.Somewhere between 20 and 45 psi✓ Answer
- D.At 100 psi
On a tractor, the spring brakes will come on automatically when air pressure drops to a range of about 20 to 45 psi.
8. In a dual air brake system, before driving you should build up air pressure and confirm the warning devices go off. What is a correct expectation for a properly working system?
- A.Air pressure should never exceed 60 psi
- B.The warning light should stay on while driving
- C.Both systems build up to the governor cut-out pressure and the low pressure warning shuts off by about 60 psi✓ Answer
- D.Only one tank needs to reach 60 psi before driving
A dual system has separate air pressure for each half; both should build to cut-out and the low-pressure warning should turn off by about 60 psi.
9. When checking air brake system leakage with the engine off, brakes released, after the pressure has built up, what is the maximum allowable air loss for a straight truck or bus (static test)?
- A.Less than 10 psi in one minute
- B.Up to 5 psi in one minute
- C.Less than 2 psi in one minute✓ Answer
- D.Less than 2 psi in ten minutes
With brakes released (static test), a single vehicle should lose less than 2 psi of air in one minute.
10. When testing air leakage with the brakes applied (applied leakage / service brake test) on a single vehicle, what is the maximum allowable air loss?
- A.Less than 3 psi in one minute✓ Answer
- B.Less than 10 psi in one minute
- C.Less than 3 psi in five minutes
- D.Less than 2 psi in one minute
With a full application of the brakes on a single vehicle, air loss should be less than 3 psi in one minute (4 psi for a combination).
11. What is the proper way to make a normal stop with an air brake system?
- A.Stab the pedal fully then release repeatedly on dry roads
- B.Pump the brake pedal rapidly the entire time
- C.Apply the parking brake to make every stop
- D.Push the brake pedal down steadily, controlling pressure so the vehicle comes to a smooth, safe stop✓ Answer
For a normal stop, apply the brakes with steady, controlled pressure, easing off as the vehicle slows to stop smoothly.
12. On air brakes with manual slack adjusters, how much should the pushrod move when you pull hard on it (brakes released), indicating the brakes need adjustment?
- A.More than four inches is acceptable
- B.More than about one inch of movement means they likely need adjustment✓ Answer
- C.Movement of a quarter inch means they must be replaced
- D.Any movement at all means they are fine
If a slack adjuster's pushrod moves more than about one inch when pulled by hand, the brakes likely need adjustment.
13. What is the job of the air compressor governor in an air brake system?
- A.It drains water and oil from the air tanks
- B.It applies the spring brakes when pressure is lost
- C.It warns the driver when air pressure is low
- D.It controls when the compressor pumps air into the tanks✓ Answer
The governor (调速器) controls when the air compressor pumps air into the tanks, cutting out at high pressure and cutting back in when pressure drops.
14. At about what pressure does the governor normally allow the compressor to start pumping again (cut-in)?
- A.About 100 psi (cut-in)✓ Answer
- B.About 60 psi (cut-in)
- C.About 150 psi (cut-in)
- D.About 20 psi (cut-in)
The governor 'cut-in' pressure is typically around 100 psi, where it allows the compressor to resume pumping; cut-out is around 125 psi.
15. What is the purpose of the air dryer in an air brake system?
- A.To increase the maximum air pressure in the tanks
- B.To warm the air so it does not freeze
- C.To apply the parking brakes automatically
- D.To remove moisture and some oil from the compressed air before it reaches the tanks✓ Answer
The air dryer (空气干燥器) collects and removes moisture and some oil from the compressed air before it reaches the tanks.
16. In a dual air brake system, how quickly should the air compressor build pressure from 85 to 100 psi with the engine at operating rpm?
- A.Within 3 minutes
- B.It does not matter how long it takes
- C.Within 2 seconds
- D.Within 45 seconds✓ Answer
In a typical dual air system, pressure should build from 85 to 100 psi within 45 seconds with the engine at operating rpm.
17. What is the purpose of the tractor protection valve?
- A.To drain the trailer air tanks automatically
- B.To increase braking power on the trailer only
- C.To keep air in the tractor brake system if the trailer breaks away or a bad leak occurs✓ Answer
- D.To warm the service line in cold weather
The tractor protection valve (拖车保护阀) keeps air in the tractor brake system if the trailer breaks away or develops a bad leak.
18. At what air pressure will the tractor protection valve typically close automatically (control valve pops out)?
- A.At about 5 psi
- B.At exactly 100 psi
- C.In the range of about 20 to 45 psi✓ Answer
- D.Only at 0 psi
The tractor protection valve closes automatically when air pressure falls into the range of about 20 to 45 psi, sealing off the trailer lines.
19. What is the function of the brake chamber and slack adjuster together in an air brake?
- A.They store extra air for emergencies
- B.They warn the driver of low air pressure
- C.They dry the air before it reaches the tanks
- D.They convert air pressure into mechanical force that turns the S-cam to apply the brakes✓ Answer
Air pushes the brake chamber (制动气室) pushrod, which moves the slack adjuster (间隙调整器) to turn the S-cam and press the shoes against the drum.
20. What is brake lag in an air brake system?
- A.The distance a vehicle travels before the brakes are fully applied because it takes time for air to flow through the lines✓ Answer
- B.The time it takes to drain the air tanks
- C.The delay before the compressor cuts in
- D.The distance needed to build 100 psi
Brake lag (刹车滞后) is the added distance a vehicle travels because air takes about half a second to flow through the lines before the brakes fully apply.
21. The total stopping distance for an air brake vehicle is made up of which components?
- A.Only braking distance
- B.Perception distance, reaction distance, and braking distance only
- C.Perception distance, reaction distance, brake lag distance, and effective braking distance✓ Answer
- D.Reaction distance and brake lag distance only
For air brakes, total stopping distance adds a brake lag distance to perception, reaction, and effective braking distances.
22. What is the recommended braking technique when going down a long, steep grade with air brakes?
- A.Ride the brakes lightly the whole way down
- B.Apply full brake pressure continuously
- C.Leave the truck in neutral and coast
- D.Select a low gear and use light, steady brake pressure, releasing when speed is under control✓ Answer
On a long grade, use engine braking with a low gear plus light, steady brake applications to control speed and avoid overheating the brakes.
23. Which two lines connect a tractor to a trailer in an air brake combination vehicle?
- A.The intake line and the exhaust line
- B.The fuel line and the coolant line
- C.The service (control) line and the supply (emergency) line✓ Answer
- D.The primary line and the backup line only
Combination vehicles use a service line (行车管路), which carries the driver's brake application, and a supply/emergency line (供气管路) that supplies air to the trailer.
24. What happens to the trailer brakes if the supply (emergency) line loses air pressure?
- A.Nothing happens until the driver presses the pedal
- B.The trailer accelerates on its own
- C.The trailer brakes are permanently disabled
- D.The trailer's emergency (spring) brakes come on automatically✓ Answer
If the supply/emergency line loses pressure, the trailer's emergency spring brakes apply automatically to help stop the trailer.
25. What does the trailer hand valve (trolley/Johnson bar) do, and why should you not use it for parking?
- A.It applies only the trailer brakes; it should not be used for parking because it works on air pressure that can leak off✓ Answer
- B.It drains the trailer tanks; parking with it floods the lines
- C.It applies only the tractor brakes; it is fine for parking
- D.It releases the spring brakes; parking with it is required
The trailer hand valve (拖车手控阀) applies only the trailer brakes using air pressure; since air can leak off, use the parking (spring) brakes to park instead.
26. Why should you not use the spring (parking) brakes if the brakes are very hot after coming down a steep grade, or if they are wet in freezing weather?
- A.Hot brakes can be damaged and wet brakes can freeze so the vehicle cannot move✓ Answer
- B.The spring brakes will release the air tanks
- C.It will overcharge the air compressor
- D.It always sets off the low-pressure warning
Applying spring brakes on very hot brakes can damage them, and setting them on wet brakes in freezing weather can freeze them so the vehicle cannot move.
27. When making the applied leakage (service brake) test on a combination vehicle, what is the maximum allowable air loss?
- A.Less than 2 psi in one minute
- B.Less than 3 psi in one minute
- C.Less than 4 psi in one minute✓ Answer
- D.Less than 6 psi in one minute
With the brakes fully applied on a combination vehicle, air loss should be less than 4 psi in one minute (3 psi for a single vehicle).
28. When making the static leakage test on a combination vehicle (engine off, brakes released), what is the maximum allowable air loss?
- A.Less than 3 psi in one minute✓ Answer
- B.Less than 2 psi in one minute
- C.Less than 5 psi in one minute
- D.Less than 3 psi in five minutes
With brakes released (static test), a combination vehicle should lose less than 3 psi in one minute (2 psi for a single vehicle).
29. During a pre-trip check, how do you test that the low air pressure warning signal works?
- A.Drain all the tanks completely before starting the engine
- B.With enough pressure, turn off the engine, turn the key to on, and step on and off the brake to reduce pressure until the warning comes on✓ Answer
- C.Rev the engine until the compressor cuts out
- D.Disconnect the trailer supply line
With the key on and engine off, fan the brakes to lower air pressure; the low air pressure warning should come on before pressure drops below 60 psi.
30. During a pre-trip inspection, how should you check the air compressor drive belt (if the compressor is belt-driven)?
- A.Confirm the belt is missing so the compressor can free-spin
- B.Soak the belt in oil before each trip
- C.Cut a small notch in the belt to test flexibility
- D.Check that the belt is in good condition and properly tight✓ Answer
If the air compressor is belt-driven, check that the drive belt is in good condition and properly adjusted (tight) so the compressor can keep up air pressure.
31. What is the purpose of the alcohol evaporator in some air brake systems?
- A.To increase engine power
- B.To help reduce the risk of ice in the air brake valves in cold weather✓ Answer
- C.To replace the need to drain the tanks entirely
- D.To boost tank pressure above 150 psi
The alcohol evaporator (酒精蒸发器) puts alcohol into the air system to help reduce the risk of ice in air brake valves in cold weather; you still must drain tanks.
32. What should you do daily with air tanks that do not have automatic drain valves?
- A.Nothing; they never collect water
- B.Add oil to each tank
- C.Manually open the drain valves to drain water and oil from each tank✓ Answer
- D.Increase the pressure to 150 psi to blow them clear
Tanks without automatic drains must be drained manually each day by opening each drain valve to remove accumulated water and oil.
33. In a dual air brake system, what are the two separate air systems usually called?
- A.The intake system and the exhaust system
- B.The front system and the parking system
- C.The wet system and the alcohol system
- D.The primary system and the secondary system✓ Answer
A dual air brake system has a primary system (主系统) and a secondary system (副系统), each with its own tanks, so one failing still leaves braking on some axles.
34. If the low air pressure warning comes on while driving, what should you do?
- A.Ignore it as long as the brakes still work
- B.Speed up to reach your destination faster
- C.Stop and safely park the vehicle as soon as possible✓ Answer
- D.Pump the brakes repeatedly to build pressure
A low air pressure warning means air is being lost faster than it can be replaced; stop and safely park as soon as possible before the brakes fail.
35. Why should you not leave your foot on the brake pedal (fanning or riding the brakes) while going down a long grade?
- A.It can overheat the brakes and cause brake fade, and fanning can use up air faster than the compressor replaces it✓ Answer
- B.It charges the air tanks too quickly
- C.It disables the parking brakes permanently
- D.It has no downside at all
Continuous or repeated braking overheats the brakes causing brake fade, and excessive fanning can use air faster than the compressor can replace it.
36. What does the foot (treadle) valve do in an air brake system?
- A.It releases the spring brakes
- B.It lets air out of the tanks to apply the service brakes in proportion to how hard you press✓ Answer
- C.It drains water from the reservoirs
- D.It controls the air compressor governor
The foot (treadle) valve (脚控阀) applies the service brakes; pressing harder sends more air pressure to the brakes, and letting up reduces it.
37. How does releasing the brake pedal too much on a long downgrade affect your air supply?
- A.It has no effect on the air supply
- B.Each brake application uses air, so too many light on-off applications can drain the tanks faster than the compressor refills them✓ Answer
- C.It permanently seals the air tanks
- D.It cools the compressor so it stops working
Every brake application uses air; repeatedly applying and releasing (fanning) can use air faster than the compressor can replace it, lowering tank pressure.
38. What are the reservoirs (air tanks) for in an air brake system?
- A.To store diesel fuel for emergencies
- B.To hold engine coolant
- C.To store enough compressed air so the brakes can be used several times even if the compressor stops✓ Answer
- D.To generate air pressure by themselves
Reservoirs (储气罐) store enough compressed air so the brakes can be applied several times even if the compressor stops working.
39. What is one advantage of disc brakes compared with S-cam drum brakes on air brake systems?
- A.They require no air pressure at all
- B.They generally provide more effective braking and are less prone to some drum-brake issues✓ Answer
- C.They never need any inspection
- D.They eliminate the need for a brake pedal
In air disc brakes (盘式刹车), a caliper squeezes a rotor with pads; they generally give effective braking and avoid some drum-brake shortcomings, but still use air pressure.
40. Which gauge tells you how much air pressure the driver is applying to the brakes?
- A.The air pressure (supply) gauge
- B.The coolant temperature gauge
- C.The application pressure gauge✓ Answer
- D.The oil pressure gauge
The application pressure gauge (施加压力表) shows how much air pressure is being applied to the brakes; rising application pressure to hold speed can signal brake fade.
41. Besides a warning light, what other low air pressure warning device may an older air brake vehicle have?
- A.A wig wag that drops into the driver's view✓ Answer
- B.A flashing headlight
- C.An automatic horn on the trailer
- D.A GPS alert
Some older vehicles have a 'wig wag' — a mechanical arm that drops into the driver's view when pressure falls below about 60 psi, in addition to a warning light or buzzer.
42. What is the correct way to use the parking brake control (yellow diamond-shaped knob) to park a straight truck?
- A.Push the knob in to set the parking brakes
- B.Pull the knob out to set the parking brakes✓ Answer
- C.Turn the knob halfway to hold pressure
- D.Leave the knob in and rely on the service brakes
The yellow diamond-shaped knob controls the spring parking brakes; you pull it out to apply (set) the parking brakes and push it in to release them.
43. What does the red eight-sided knob control on a tractor with air brakes?
- A.The trailer air supply (tractor protection) valve✓ Answer
- B.The windshield wipers
- C.The engine fuel shutoff
- D.The trailer lights
The red eight-sided knob is the trailer air supply valve; pushing it in supplies air to the trailer, and it pops out to shut off trailer air (tractor protection valve).
44. Why is it important that manual slack adjusters be checked and kept in proper adjustment?
- A.Out-of-adjustment brakes are a leading cause of poor braking and are illegal; brakes may not stop the vehicle safely✓ Answer
- B.It only affects fuel economy
- C.It changes the tank pressure setting
- D.It has no real effect on stopping
Brakes out of adjustment are a leading cause of poor braking and are illegal; if pushrod travel is too long the brakes may not stop the vehicle safely.
45. After a full charge, what test confirms the tractor protection valve and trailer emergency brakes work correctly on a combination?
- A.Draining the tanks to zero and checking the compressor
- B.Fanning down pressure so the trailer air supply/tractor protection valve pops out (usually 20 to 45 psi), setting the trailer emergency brakes✓ Answer
- C.Revving the engine to 150 psi
- D.Turning the key off and coasting
Fanning the brakes to lower pressure should make the trailer air supply / tractor protection valve pop out in the 20 to 45 psi range, confirming the trailer emergency brakes set.
46. Compared with a vehicle at 55 mph, how does air-brake lag change your total stopping distance?
- A.It shortens total stopping distance
- B.It removes the need for reaction distance
- C.It has no measurable effect
- D.It adds roughly 32 feet at 55 mph because of the time air takes to work✓ Answer
Air brake lag adds distance; at 55 mph the roughly half-second lag adds about 32 feet to your total stopping distance compared with hydraulic brakes.
47. During inspection, if you find oil or water constantly leaking or spraying from the air system, what does this indicate?
- A.The system is working perfectly and needs nothing
- B.A defect that should be repaired before driving, since it can affect braking✓ Answer
- C.That you should add more oil to the tanks
- D.That the alcohol evaporator is overfull
Constant oil or water leaking/spraying signals a defect in the air brake system that must be repaired before driving, as it can reduce braking effectiveness.
48. What is the main reason a dual air brake system is safer than a single air brake system?
- A.It uses less air overall
- B.It never needs the tanks drained
- C.If one system loses air, the other still provides braking on some axles✓ Answer
- D.It removes the need for spring brakes
In a dual system, the primary and secondary circuits are independent, so if one loses air the other still provides braking on some axles, letting you stop.
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 →