20 Brake Hydraulics & Power Assist Practice Questions & Answers

1. 1. During scheduled maintenance the brake master cylinder reservoir is found below the MIN mark. What is the correct action at the maintenance and light repair level?

   • A.Add fluid and pump the pedal firmly several times to reseat the master cylinder cup seals
   • B.Add whatever brake fluid is already open on the bench and note the low level on the repair order
   • C.Fill the reservoir to the top of the neck so the level stays above MIN until the next service
   • D.Check the system for external leakage and check lining wear, then add the specified fluid✓ Answer

   Fluid level falls normally as friction material wears and the caliper pistons take up the gap, but it also falls because of an external leak, so a low reservoir is a finding to be investigated before any fluid is added. Filling above the MAX mark also leaves no room for the fluid pushed back into the reservoir when the pistons are retracted at the next lining service.

   Source: Official ASE Study Guide, Automobile Maintenance and Light Repair (G1) Test — Brakes: checking master cylinder fluid level and condition and inspecting for external leakageReport a problem with this question

2. 2. Why must brake fluid be kept in a tightly closed container, and why is a container that has been left open no longer suitable for use?

   • A.Brake fluid separates into layers in open air and has to be stirred before use
   • B.Brake fluid evaporates in open air, and the loss of volume thickens what is left
   • C.Brake fluid gives off vapors that can be ignited by an open flame in the shop
   • D.Brake fluid draws moisture from the air, and water lowers its boiling point✓ Answer

   Glycol based brake fluid is hygroscopic: it pulls water vapor out of the air and holds it in solution. The absorbed water sharply lowers the fluid's boiling point, so heat from hard braking can boil the fluid and put compressible vapor in the lines, producing a fading or sinking pedal.

   Source: Official ASE Study Guide, Automobile Maintenance and Light Repair (G1) Test — Brakes: selecting, handling, storing and installing proper brake fluid; FMVSS 116 wet and dry boiling point requirementsReport a problem with this question

3. 3. A vehicle's service information calls for a glycol based brake fluid. Which statement about brake fluid grades is correct?

   • A.DOT 5 is a glycol grade that is approved only for systems with drum brakes
   • B.DOT 5 is silicone based and is not mixed with the glycol based grades✓ Answer
   • C.DOT 5 and DOT 5.1 are the same fluid and the two names are used interchangeably
   • D.DOT 5 is a mineral oil grade used where the system has no rubber seals

   DOT 3, DOT 4 and DOT 5.1 are polyalkylene glycol ether fluids, while DOT 5 is silicone. Silicone does not blend with glycol fluid, aerates readily and is difficult to bleed, so it is never poured into a system built for a glycol grade. The technician installs the grade the vehicle manufacturer specifies.

   Source: FMVSS 116, Motor Vehicle Brake Fluids — DOT 3, DOT 4, DOT 5 and DOT 5.1 designationsReport a problem with this question

4. 4. All of the following findings during a flexible brake hose inspection require replacing the hose EXCEPT:

   • A.A bulge that appears in the hose while an assistant holds the brake pedal down
   • B.Chafing that has worn through the outer cover and exposed the reinforcement
   • C.A coating of road dust on a hose that is otherwise dry, sound and flexible✓ Answer
   • D.Cracks in the outer cover that are deep enough to reach the braided layer

   The outer cover protects the braided reinforcement that carries the pressure. A bulge under pedal pressure proves that reinforcement has already failed, and chafing or cracking that reaches the braid has destroyed the protection, so all three conditions condemn the hose. Surface dirt on a dry, flexible hose is simply wiped away.

   Source: Official ASE Study Guide, Automobile Maintenance and Light Repair (G1) Test — Brakes: inspecting flexible hoses and fittings for leaks, dents, kinks, rust, cracks and wearReport a problem with this question

5. 5. A steel brake line is found with a deep dent and a kink. How is that line corrected?

   • A.Replace the damaged section with steel brake tubing and properly formed flares✓ Answer
   • B.Wrap the damaged area and add a clamp so the dented section cannot flex
   • C.Straighten the kink and reuse the line if it holds pressure during a bleed
   • D.Splice in a length of copper tubing using a compression union at each joint

   A dent or kink has already thinned and work hardened the tube wall, so the line cannot be straightened and returned to service. Replacement uses brake tubing of the same size and material with double or ISO flares and the correct fittings; ordinary copper tubing and compression unions are not rated for brake system pressure.

   Source: Official ASE Study Guide, Automobile Maintenance and Light Repair (G1) Test — Brakes: inspecting brake lines, fittings and supports; FMVSS 106 brake hose and tubing requirementsReport a problem with this question

6. 6. With the engine off, a technician pumps the brake pedal several times to use up reserve vacuum, then holds steady pressure on the pedal and starts the engine. Which result shows that the vacuum booster is working?

   • A.The pedal stays exactly where it is held while the idle speed settles down
   • B.The pedal becomes noticeably harder and its travel shortens at the same time
   • C.The pedal drops slightly away under the foot once the engine starts and idles✓ Answer
   • D.The pedal rises against the foot as manifold vacuum builds in the booster

   Pumping the pedal exhausts the stored vacuum so both sides of the diaphragm sit at atmospheric pressure and the pedal is hard. When the engine starts, manifold vacuum is applied to the front chamber and the pressure difference pushes the diaphragm and pushrod forward, so the held pedal moves down. No movement means no assist is being produced.

   Source: Official ASE Study Guide, Automobile Maintenance and Light Repair (G1) Test — Power assist units: testing pedal free travel with and without the engine runningReport a problem with this question

7. 7. What is verified when the vacuum check valve at a brake booster is inspected?

   • A.That it opens in both directions so the booster follows manifold vacuum
   • B.That it lets air into the booster so the pressure equalizes after every stop
   • C.That it limits vacuum so the booster gives full assist only above idle speed
   • D.That it passes air in one direction only and holds vacuum in the booster✓ Answer

   The check valve lets air move out of the booster toward the vacuum source but not back in. That one way action traps the vacuum, so the booster keeps a reserve for assisted stops when manifold vacuum falls off at wide open throttle or when the engine stops. A valve that passes both ways loses the reserve and the pedal goes hard.

   Source: Official ASE Study Guide, Automobile Maintenance and Light Repair (G1) Test — Power assist units: inspecting the vacuum booster and check valve for operation and leaksReport a problem with this question

8. 8. A light truck uses a hydro-boost power assist unit. Technician A says the assist comes from power steering pump pressure, so low or aerated power steering fluid also reduces brake assist. Technician B says the unit's accumulator stores enough energy for a limited number of assisted stops after the engine has stopped running. Who is right?

   • A.Both Technician A and Technician B✓ Answer
   • B.Neither Technician A nor Technician B
   • C.Technician A only
   • D.Technician B only

   A hydro-boost unit takes hydraulic pressure from the power steering pump and meters it through a spool valve to boost the master cylinder, so any power steering fault — low level, aerated fluid, a weak pump, a restricted hose — shows up as reduced brake assist. Its accumulator stores pressure so a few assisted applications remain available if the engine quits.

   Source: Official ASE Study Guide, Automobile Maintenance and Light Repair (G1) Test — Power assist units: identifying hydro-boost operation and checking for leaks and operationReport a problem with this question

9. 9. A vehicle has an electro-hydraulic brake assist unit with an electric pump and a pressure accumulator. What must be done before any hydraulic line on that unit is opened?

   • A.Let the engine idle so the pump keeps the accumulator at its full pressure
   • B.Cycle the ignition on and pump the pedal so the pump refills the accumulator
   • C.Set the parking brake and hold it so the accumulator pressure stays locked in
   • D.Disable the pump and relieve the stored pressure as the service information directs✓ Answer

   The accumulator on an electro-hydraulic assist unit stays charged to high pressure after the key is off, and the pump can run as soon as the system wakes up. Opening a fitting before the pump is disabled and the stored pressure is bled down the way the service information specifies releases fluid violently and risks injury.

   Source: Official ASE Study Guide, Automobile Maintenance and Light Repair (G1) Test — Power assist units: identifying and checking electro-hydraulic assist system operationReport a problem with this question

10. 10. The hydraulic system on an ABS equipped vehicle is being flushed. Why does the manufacturer specify a bleeding sequence and, on many vehicles, a scan tool bleed procedure?

    • A.The sequence lets the fluid warm enough to drive the dissolved moisture out of it
    • B.Air trapped in the modulator passages comes out when the valves are cycled✓ Answer
    • C.Bleeding out of sequence reverses the proportioning valve and locks the rear brakes
    • D.The scan tool sets the pedal height specification once the bleeding is finished

    Passages inside the ABS hydraulic control unit are closed off by solenoid valves during ordinary pedal bleeding, so air can sit there untouched and give a soft pedal no matter how long the wheels are bled. The scan tool commands those valves and the pump to cycle, moving the trapped air out, and the specified wheel order makes sure each circuit is purged completely.

    Source: Official ASE Study Guide, Automobile Maintenance and Light Repair (G1) Test — Brakes: bleeding and flushing the hydraulic system, including manufacturer sequence and scan tool proceduresReport a problem with this question

11. 11. On a road test after hydraulic service, the brake pedal slowly sinks toward the floor while steady foot pressure is held, and no leak is visible anywhere on the vehicle. What is the correct action?

    • A.Bleed the rear wheels once more, since only trapped air lets a pedal sink
    • B.Hold the vehicle and report it, because the pedal is losing pressure internally✓ Answer
    • C.Release the vehicle, since a slow sink with no visible leak is normal pedal creep
    • D.Adjust the pushrod at the booster to take the sink out of the pedal travel

    A sealed hydraulic system holds the pedal in place under steady pressure because the fluid cannot go anywhere. A pedal that keeps sinking is losing pressure past the master cylinder cup seals or through a leak that is not yet visible, which is an unsafe condition, so the vehicle is held and the finding written up rather than released.

    Source: Official ASE Study Guide, Automobile Maintenance and Light Repair (G1) Test — Brakes: checking for a high, low, hard or spongy pedal and reporting the conditionReport a problem with this question

12. 12. A brake master cylinder reservoir has been filled with a petroleum based oil instead of brake fluid. What service is required?

    • A.Add a full container of the specified fluid so the contamination is diluted
    • B.Flush the system and replace every rubber part the contaminated fluid reached✓ Answer
    • C.Drain the reservoir, refill with the specified fluid and bleed all four wheels
    • D.Bleed the system until clean fluid comes out at the wheel farthest from the master cylinder

    The cups, seals and flexible hoses in a brake system are made of rubber compounds chosen for glycol fluid; petroleum products make them swell and soften almost immediately. Flushing alone leaves the swollen parts in the system, where they can block ports or leak, so every rubber component the contaminated fluid touched is replaced.

    Source: Official ASE Study Guide, Automobile Maintenance and Light Repair (G1) Test — Brakes: identifying brake fluid contamination and required system serviceReport a problem with this question

13. 13. How is the red brake warning lamp verified after hydraulic system service?

    • A.Confirm it stays lit the whole time the engine runs so the circuit is proven live
    • B.Confirm it flashes each time the pedal is pressed to prove the switch works
    • C.Confirm it stays dark with the ignition on so no fault is stored in the module
    • D.Confirm it lights during the bulb check and lights again with the parking brake set✓ Answer

    The red lamp is fed by more than one input: the bulb check proves the bulb and its wiring, and applying the parking brake proves the parking brake switch and its circuit. The lamp must then go out with the brake fully released and the engine running; a lamp that stays on afterward is itself a finding to be reported.

    Source: Official ASE Study Guide, Automobile Maintenance and Light Repair (G1) Test — Brakes: verifying the brake warning lamp and inspecting brake system wiringReport a problem with this question

14. 14. A vehicle is in for a brake fluid flush. With the ignition on, the ABS warning lamp comes on and stays on after the engine starts. What should the technician do?

    • A.Retrieve and record the stored codes and report the finding before clearing✓ Answer
    • B.Disconnect the battery for several minutes so the ABS module resets its lamp
    • C.Bleed the system again, since air in the modulator is what turns the lamp on
    • D.Clear the codes and release the vehicle if the pedal feels normal on a road test

    A lamp that stays on after the self test means the module has found a fault, disabled anti-lock control and stored at least one code. Those codes are the only record of what happened, so they are read and written down before anything is cleared or the battery is disconnected, and the customer is told that anti-lock function is currently unavailable.

    Source: Official ASE Study Guide, Automobile Maintenance and Light Repair (G1) Test — Brakes: verifying the ABS warning lamp operation and retrieving and recording diagnostic trouble codes before clearingReport a problem with this question

15. 15. How is the parking brake indicator switch and its circuit checked?

    • A.Confirm the lamp follows the switch as the parking brake is set and released✓ Answer
    • B.Confirm the lamp stays lit with the brake released, proving the circuit is good
    • C.Confirm the lamp brightens gradually as the lever is pulled through its travel
    • D.Confirm the lamp comes on only after the vehicle is driven with the brake set

    The indicator switch is a simple on/off device that completes the lamp circuit as soon as the parking brake starts to apply and opens when it is fully released. Operating the brake through its full range while watching the lamp therefore checks the switch, its adjustment and the wiring in one step; a lamp that stays on with the brake fully off points to a misadjusted switch or a shorted circuit.

    Source: Official ASE Study Guide, Automobile Maintenance and Light Repair (G1) Test — Brakes: testing the parking brake indicator lamp, switch and wiringReport a problem with this question

16. 16. A front flexible brake hose has just been replaced. What is verified about the new hose before the vehicle is released?

    • A.That it is routed with a slight twist so it can absorb suspension movement
    • B.That it is pulled taut so it cannot flex and rub against the strut housing
    • C.That it has no twist and clears nearby parts at full lock and full travel✓ Answer
    • D.That it rests on the lower control arm so the arm carries part of its weight

    A twisted hose is stressed every time the suspension moves and can restrict flow or tear at the fitting, and a hose that touches a tire, spring or steering part will chafe through. Because the front hose has to serve both steering and suspension motion, the clearance check is made at full left and right lock and through full suspension travel.

    Source: Official ASE Study Guide, Automobile Maintenance and Light Repair (G1) Test — Brakes: inspecting flexible hoses for proper routing, supports and clearanceReport a problem with this question

17. 17. Technician A says brake fluid drained from the wheel cylinders and calipers during bleeding may be poured back into the reservoir once it has settled and cleared. Technician B says brake fluid spilled on a painted surface must be washed off right away because it attacks paint. Who is right?

    • A.Neither Technician A nor Technician B
    • B.Technician B only✓ Answer
    • C.Both Technician A and Technician B
    • D.Technician A only

    Fluid that has come out of the system is aerated and carries absorbed moisture and wear debris, so it is discarded and never returned to the reservoir no matter how clear it looks. Technician B is correct: glycol brake fluid acts as a paint stripper, so a spill is flushed with plenty of water immediately.

    Source: Official ASE Study Guide, Automobile Maintenance and Light Repair (G1) Test — Brakes: selecting, handling, storing and installing proper brake fluidReport a problem with this question

18. 18. A replacement master cylinder is being installed. Technician A says the master cylinder should be bench bled before it is mounted on the vehicle. Technician B says the reservoir should then be filled to the top of the filler neck so the level cannot drop during bleeding. Who is right?

    • A.Technician A only✓ Answer
    • B.Neither Technician A nor Technician B
    • C.Both Technician A and Technician B
    • D.Technician B only

    Bench bleeding fills the bore and outlet ports with fluid before installation, because air trapped in the master cylinder is very difficult to push out from the wheel bleeders. Technician B is wrong: the reservoir is filled to the MAX line, since an overfilled reservoir has no room for thermal expansion or for the fluid returned when caliper pistons are pushed back.

    Source: Official ASE Study Guide, Automobile Maintenance and Light Repair (G1) Test — Brakes: master cylinder service and bleeding the hydraulic systemReport a problem with this question

19. 19. Technician A says any DOT grade on the shelf can be used to top off a reservoir because all brake fluids mix. Technician B says a spongy brake pedal is corrected by lengthening the booster pushrod. Who is right?

    • A.Technician A only
    • B.Technician B only
    • C.Neither Technician A nor Technician B✓ Answer
    • D.Both Technician A and Technician B

    Brake fluid must meet the grade the vehicle manufacturer specifies; silicone DOT 5 is not compatible with the glycol grades, so the fluids do not simply mix. A spongy pedal comes from compressible air in the hydraulic system and is corrected by bleeding, not by pushrod adjustment; a pushrod that is too long holds the compensating port closed and makes the brakes drag.

    Source: Official ASE Study Guide, Automobile Maintenance and Light Repair (G1) Test — Brakes: brake fluid selection and bleeding the hydraulic system; FMVSS 116 brake fluid gradesReport a problem with this question

20. 20. A vehicle's brake booster is supplied by an auxiliary electric vacuum pump instead of intake manifold vacuum. What does the inspection of that vacuum supply include?

    • A.Confirming the pump runs only while the starter cranks and shuts off afterward
    • B.Confirming the pump vents the booster to atmosphere each time the pedal returns
    • C.Confirming the pump runs and checking its hose, fittings and check valve for leaks✓ Answer
    • D.Confirming the pump holds a steady pressure inside the booster with the key off

    An auxiliary pump is used where the engine cannot supply enough manifold vacuum, and it is controlled by a sensor or switch so it runs whenever booster vacuum falls low, not only during cranking. Assist depends on the whole supply path being sealed, so the inspection covers pump operation plus hose condition and routing, the fittings and the check valve.

    Source: Official ASE Study Guide, Automobile Maintenance and Light Repair (G1) Test — Power assist units: checking the vacuum supply from a manifold or auxiliary pump sourceReport a problem with this question

Practice questions based on the Test Specification and Task List published in the Official ASE Study Guide for the Automobile Maintenance and Light Repair (G1) test. ASE is a mark of the National Institute for Automotive Service Excellence; this site is not affiliated with or endorsed by ASE. ASE certification requires passing the test and documenting relevant work experience, and appropriate vocational training may count toward that experience — confirm current requirements with ASE before testing. No question here keys a manufacturer specification: torque values, clearances, pressures and capacities differ by vehicle, so any figure a question needs is given in the question itself, exactly as you would read it off a service manual on the job. About the ASE G1 test →