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The air brake check and the hydraulic brake check

Skills test guide5 minManual section 11.1.2

This is the most important part of the inspection test. It has a star on the checklist: if you do not explain AND perform it correctly, you fail the inspection test automatically. Air brake vehicles do a 4-part check; hydraulic brake vehicles do a pedal and warning check.

Why the air brake check matters

Air brake safety devices are not the same on every truck. The check proves that the safety devices work correctly as air pressure falls from normal to low air.

If air pressure is lost while driving, the emergency (spring) brakes come on by themselves and the vehicle stops suddenly and out of control. The warning devices give you time to stop safely before that happens.

You must explain and perform all four parts. Missing any part, or doing it wrong, is an automatic failure of the Vehicle Inspection test.

The numbers below are the common values. The manual also says "or the level specified by the manufacturer" — you are responsible for knowing and telling the examiner the maker's specifications for the vehicle you test in.

Part 1 — Air gauge and governor cut-out

  • Chock the wheels if required. Start the engine and check that the air gauge(s) work.
  • Let the air pressure build until the governor cut-out — the point where the compressor stops pumping. Tell the examiner the pressure: about 120–140 psi (or the manufacturer's level).

Say: The governor cut out at 125 psi, which is within 120 to 140 psi.

Part 2 — Air leakage rate test

  • Shut off the engine and turn the key back to "on" (battery charge) so the gauges and warnings still work.
  • Release the parking brake (all vehicles) and, on a combination, push in the tractor protection valve.
  • Fully press the foot brake. The gauge drops a little at first. After it stabilizes, hold the brake for 1 minute and watch the gauge.
  • Allowed loss after stabilizing: no more than 3 psi in 1 minute for a single vehicle (or a trailer without air brakes), no more than 4 psi in 1 minute for a combination vehicle.

Say: I'm holding the brake for one minute. The pressure dropped 2 psi, which is under the 4 psi limit for a combination vehicle.

Part 3 — Low air warning test

  • Do not restart the engine. Keep the key or power on.
  • Fan the brakes: press and release the foot brake again and again to let air out.
  • Tell the examiner the pressure where the low air warning (buzzer, light, and/or wig-wag flag) comes on. It must come on before the pressure drops below 55 psi.

Say: The low air warning came on at 60 psi, which is above 55 psi.

Part 4 — Emergency (parking) brake valve test

  • Keep fanning off the air. The parking brake (emergency brake) valve should close — pop out — between 20 and 45 psi (no less than 20, no more than 45), or at the manufacturer's level.
  • On a combination with air brakes, the tractor protection valve and the trailer parking brake valve should pop out at the same time as, or before, the truck/tractor/bus valve.
  • If your valves are not designed to pop out, listen for the sound of air discharging — that sound shows the spring brakes have applied.
  • If the vehicle has a primary and secondary (dual) air system, explain that there is a backup system, so the valve(s) will not pop out when only one of the two systems loses pressure.

Say: The parking brake valve popped out at 35 psi, which is between 20 and 45 psi.

Hydraulic brake check (hydraulic brake vehicles only)

Damaged hoses, leaks or low brake fluid can cause partial or total loss of braking, longer stopping distances, or a crash. This check is also starred: explain and perform it correctly or you fail automatically. Systems differ, so follow your vehicle maker's steps:

  • Check that the brake indicators and warning systems work, if equipped.
  • With the transmission in park, point out the normal height of the brake pedal. Then test pedal pressure: pump the pedal 3 times, then hold it down for 5 seconds (or the maker's method).
  • The pedal should not sink after the first application. It should feel firm and keep enough reserve (it should not go to the floor).
  • Check that the hydraulic power assist system works, if equipped.

Some systems need the key on and/or the engine running for this check. Explain and show the steps for your vehicle.

Say: I pumped the brake pedal three times and I'm holding it for five seconds. The pedal is firm and does not move down.

Check your state

Some state manuals print only "120–140 psi" for governor cut-out, while others add "or the level specified by the manufacturer." Know your own vehicle's numbers and check your state manual.

References and scope

The topic reference is listed below. Sources support verification; the lesson itself is here. The current content needs technical and language review before public launch.

Manual reference: 11.1.2

Official CDL handbook · Utah ↗StatesOur editorial approach

Put it into practice

Educational questions with explanations. The answer review also works without the interactive quiz.

Preparing your questions…

Review questions with answers

  1. At about what pressure should the air compressor governor cut out?
    1. 60–80 psi
    2. 90–100 psi
    3. 120–140 psi
    4. 150–175 psi
    Show answer
    120–140 psi

    Build air to governor cut-out, which happens at about 120–140 psi (or the manufacturer's level).

  2. In the air leakage rate test on a combination vehicle, after the gauge stabilizes the pressure should drop no more than:
    1. 2 psi in 1 minute
    2. 4 psi in 1 minute
    3. 6 psi in 1 minute
    4. 10 psi in 1 minute
    Show answer
    4 psi in 1 minute

    Combination vehicles may lose no more than 4 psi in one minute with the brake fully applied.

  3. For a single vehicle (or one with a trailer without air brakes), the maximum leak rate with the brake applied is:
    1. 3 psi in 1 minute
    2. 4 psi in 1 minute
    3. 5 psi in 1 minute
    4. 1 psi in 1 minute
    Show answer
    3 psi in 1 minute

    A single vehicle may lose no more than 3 psi in one minute after the gauge stabilizes.

  4. How should the engine and key be set for the air leakage rate test?
    1. Engine running at high idle
    2. Engine off and key removed
    3. Engine running, key off
    4. Engine off, key turned back to "on"
    Show answer
    Engine off, key turned back to "on"

    Shut off the engine and return the key to "on" (battery charge) so gauges and warnings work.

  5. The low air warning device must come on before air pressure drops below:
    1. 20 psi
    2. 55 psi
    3. 75 psi
    4. 100 psi
    Show answer
    55 psi

    The buzzer, light and/or flag must activate before pressure falls below 55 psi.

  6. As you fan off the air, the parking (emergency) brake valve should pop out between:
    1. 5 and 15 psi
    2. 55 and 65 psi
    3. 20 and 45 psi
    4. 60 and 90 psi
    Show answer
    20 and 45 psi

    The valve should close (pop out) no lower than 20 psi and no higher than 45 psi, or the maker's level.

  7. What is the result of not explaining and performing all four parts of the air brake check correctly?
    1. Automatic failure of the Vehicle Inspection test
    2. A small point deduction
    3. An L restriction only
    4. The examiner does it for you
    Show answer
    Automatic failure of the Vehicle Inspection test

    The air brake check is starred: any error means automatic failure of the inspection test.

  8. On a combination vehicle, the tractor protection valve and trailer parking brake valve should pop out:
    1. Only after the tractor valve
    2. Never
    3. Above 100 psi
    4. At the same time as, or before, the tractor's valve
    Show answer
    At the same time as, or before, the tractor's valve

    The trailer valves should close at the same time as or before the truck/tractor valve.

  9. If the vehicle's brake valves are not designed to pop out, how do you know the spring brakes applied?
    1. The horn sounds
    2. You hear air discharging from the system
    3. The engine stops
    4. The ABS light turns green
    Show answer
    You hear air discharging from the system

    Listen for the sound of air discharging, which shows the parking/emergency brakes have applied.

  10. True or false: On a vehicle with a primary and secondary air system, the valves may not pop out when only one system loses pressure, and you should explain this backup to the examiner.
    1. True
    2. False
    Show answer
    True

    With dual systems, losing pressure in one system will not make the valves pop out; explain the backup system.

  11. During the low air warning test, how do you lower the air pressure?
    1. Open the drain valves
    2. Restart the engine
    3. Fan the brakes by pressing and releasing the foot brake
    4. Disconnect the air lines
    Show answer
    Fan the brakes by pressing and releasing the foot brake

    Without restarting the engine, apply and release the foot brake repeatedly to fan off the air.

  12. In the hydraulic brake check, how do you test brake pedal pressure?
    1. Press it once lightly
    2. Hold it down for 1 minute with the engine off
    3. Pump it 10 times quickly
    4. Pump it 3 times, then hold it down for 5 seconds
    Show answer
    Pump it 3 times, then hold it down for 5 seconds

    Pump the pedal three times, then hold it for five seconds (or as the manufacturer specifies).

  13. During the hydraulic brake check, a good brake pedal should:
    1. Slowly sink to the floor
    2. Stay firm and not move down after the first application
    3. Feel soft and spongy
    4. Pulse up and down
    Show answer
    Stay firm and not move down after the first application

    The pedal should be firm, not depress after the initial application, and have enough reserve.

  14. Before starting the leakage test on a combination vehicle, you release the parking brake and:
    1. Push in the tractor protection valve
    2. Pull out the trailer air supply
    3. Drain the air tanks
    4. Disconnect the glad hands
    Show answer
    Push in the tractor protection valve

    Release the parking brake, and on a combination also push in the tractor protection valve; then press the foot brake all the way.