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Step 7: the final air brake check, test by test

Air Brakes5 minManual section 5.3.3

On an air brake vehicle, Step 7 of the pre-trip inspection is a set of air brake tests done instead of the hydraulic brake check. These tests and their numbers are some of the most tested facts on the Air Brakes exam. Learn each test, how to do it, and the pass/fail number.

1. Governor cut-in and cut-out

The compressor should start pumping (cut-in) at about 100 psi and stop (cut-out) at about 125 psi. Always check the manufacturer's specifications.

  • Cut-out test: run the engine at a fast idle. The governor should cut out the compressor at about the specified pressure. The gauge needle stops rising.
  • Cut-in test: with the engine idling, step on and off the brake to lower tank pressure. The compressor should cut in at about the specified pressure, and the pressure begins to rise.

If the governor does not work this way, it may need repair. A bad governor may not keep enough air pressure for safe driving.

2. Air leakage rate

Brakes released (static) test: with a fully charged system (typically 125 psi), turn off the engine, push in the parking brake to release it, and time how fast pressure drops. The loss should be less than 2 psi in one minute for single vehicles and less than 3 psi in one minute for combination vehicles.

Brakes applied test: with pressure built up to governor cut-out (120–140 psi), shut off the engine, chock the wheels (if needed), release the parking brake on any vehicle, release the tractor protection valve on a combination, and fully apply the foot brake. Hold it for one minute and watch the gauge.

With the foot brake held, air loss is too much if pressure falls more than 3 psi in one minute (single vehicle) or more than 4 psi in one minute (combination vehicle).

If the loss is too much, find and fix the leaks before driving. Otherwise you could lose your brakes while driving.

3. Low pressure warning signal

Shut the engine off when you have enough air pressure that the warning is not on. Switch the electrical power on, then press and release the brake pedal again and again to bring tank pressure down (this is called fanning the brakes).

The low air warning must come on before pressure drops below 55 psi in the air tank — in dual systems, in the tank with the lowest pressure.

If the warning does not work, you could lose air pressure without knowing it. In a single-circuit system this could cause sudden emergency braking. In dual systems, stopping distance will be longer, and only limited braking is possible before the spring brakes come on.

4. Spring brakes come on automatically

Keep fanning off air pressure by stepping on and off the brake pedal.

  • On a tractor-trailer, the tractor protection valve and the parking brake valve should close (pop out).
  • On other combination vehicles and single vehicles, the parking brake valve should close (pop out).

This should happen when pressure falls to the manufacturer's specification, 20–45 psi, and it makes the spring brakes come on.

5. Rate of air pressure buildup

Dual air systems: with the engine at operating rpm, pressure should build from 85 to 100 psi within 45 seconds. If the vehicle has larger than minimum air tanks, buildup can take longer and still be safe — check the manufacturer's specifications.

Single air systems (pre-1975): the usual standard is 50 to 90 psi within 3 minutes, with the engine idling at 600–900 rpm.

If air pressure does not build up fast enough, pressure may drop too low while driving, forcing an emergency stop. Do not drive until the problem is fixed.

6. Parking brake test and 7. Service brake test

Parking brake: stop, set the parking brake, and pull gently against it in a low gear to be sure it holds.

Service brakes: wait until air pressure is normal, release the parking brake, roll forward slowly (about 5 mph), and brake firmly with the pedal. Notice any pulling to one side, unusual feel, or delayed stopping action.

This test can show you problems you would otherwise not find until you needed the brakes on the road.

Quick review of the numbers

  • Governor: cut-in about 100 psi, cut-out about 125 psi.
  • Leakage, brakes released: less than 2 psi/min single, less than 3 psi/min combination.
  • Leakage, brakes applied: no more than 3 psi/min single, 4 psi/min combination.
  • Low-air warning: on before 55 psi.
  • Spring brakes / valves pop out: 20–45 psi.
  • Buildup: 85→100 psi in 45 seconds (dual); 50→90 psi in 3 minutes at 600–900 rpm (single, pre-1975).
  • Service brake test: about 5 mph.

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: 5.3.3

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. On a vehicle with air brakes, the final air brake check is done:
    1. Instead of the hydraulic brake check in Step 7
    2. Only once a month
    3. Only by a mechanic
    4. After the trip is over
    Show answer
    Instead of the hydraulic brake check in Step 7

    Air brake checks replace the hydraulic brake check in Step 7 of the inspection.

  2. To check the governor cut-out pressure, you should:
    1. Pump the brake pedal with the engine off
    2. Release the parking brake and time the pressure drop
    3. Run the engine at a fast idle and note where the pressure stops rising
    4. Drive at 5 mph and apply the brakes
    Show answer
    Run the engine at a fast idle and note where the pressure stops rising

    At fast idle, the governor should cut out at about the specified pressure, and the gauge stops rising.

  3. To check the governor cut-in pressure with the engine idling, you:
    1. Pull the parking brake knob
    2. Step on and off the brake to lower tank pressure and watch where the pressure begins to rise
    3. Turn off the electrical power
    4. Open the tank drains
    Show answer
    Step on and off the brake to lower tank pressure and watch where the pressure begins to rise

    Fan the brakes to lower pressure; the compressor should cut in at about the specified pressure.

  4. During the air brake check, the compressor should start pumping at about ___ and stop at about ___.
    1. 55 psi; 100 psi
    2. 85 psi; 150 psi
    3. 20 psi; 45 psi
    4. 100 psi; 125 psi
    Show answer
    100 psi; 125 psi

    The compressor should cut in near 100 psi and cut out near 125 psi (check the maker's specs).

  5. Why is a governor that does not work properly a problem?
    1. It may not keep enough air pressure for safe driving
    2. It makes the ABS lamp come on
    3. It locks the steering
    4. It turns on the brake lights
    Show answer
    It may not keep enough air pressure for safe driving

    A bad governor may fail to keep enough air pressure for safe driving.

  6. Static leakage test (brakes released, engine off, fully charged system): the loss rate for a single vehicle should be less than:
    1. 1 psi in one minute
    2. 2 psi in one minute
    3. 3 psi in one minute
    4. 4 psi in one minute
    Show answer
    2 psi in one minute

    With brakes released, loss should be under 2 psi/min for single vehicles and under 3 psi/min for combinations.

  7. Static leakage test with brakes released: for a combination vehicle, the loss rate should be less than:
    1. 1 psi in one minute
    2. 2 psi in one minute
    3. 3 psi in one minute
    4. 5 psi in one minute
    Show answer
    3 psi in one minute

    Combination vehicles: less than 3 psi in one minute with brakes released.

  8. Applied leakage test: you hold the foot brake fully applied for one minute. For a single vehicle, the air loss is too much if pressure drops more than:
    1. 1 psi
    2. 2 psi
    3. 5 psi
    4. 3 psi
    Show answer
    3 psi

    With the brake applied, more than 3 psi in one minute (single) or 4 psi (combination) is too much.

  9. Applied leakage test on a combination vehicle: the air loss rate is too much if pressure falls more than ___ in one minute.
    1. 4 psi
    2. 2 psi
    3. 3 psi
    4. 6 psi
    Show answer
    4 psi

    For combination vehicles with the foot brake applied, more than 4 psi in one minute is too much.

  10. For the applied leakage test on a combination vehicle, which must be released before you fully apply the foot brake?
    1. Only the trailer hand valve
    2. The parking brake and the tractor protection valve
    3. The tank drains
    4. The alcohol evaporator
    Show answer
    The parking brake and the tractor protection valve

    On a combination vehicle you release both the parking brake and the tractor protection valve, then apply the foot brake.

  11. If the air loss rate is too much during the leakage test, you should:
    1. Drive slowly to your destination
    2. Add alcohol to the system
    3. Check for air leaks and fix them before driving
    4. Raise the governor cut-out setting
    Show answer
    Check for air leaks and fix them before driving

    Too much air loss means you could lose your brakes while driving. Find and fix the leaks first.

  12. How do you test the low pressure warning signal?
    1. Run the engine at fast idle until the light comes on
    2. Pull the parking knob and watch the gauge
    3. Drain one tank with the engine running
    4. Shut off the engine with enough air that the warning is off, turn the electrical power on, and fan the brake pedal
    Show answer
    Shut off the engine with enough air that the warning is off, turn the electrical power on, and fan the brake pedal

    With engine off and power on, step on and off the pedal to lower pressure; the warning must come on before 55 psi.

  13. In a dual air system, the low air warning must come on before pressure drops below 55 psi in:
    1. The tank with the lowest air pressure
    2. The primary tank only
    3. The secondary tank only
    4. The trailer tank
    Show answer
    The tank with the lowest air pressure

    In dual systems, watch the tank with the lowest pressure; the warning must come on before it drops below 55 psi.

  14. If the low air warning signal does not work on a single-circuit air system, what could happen?
    1. The compressor will overheat
    2. You could lose air pressure without knowing it, causing sudden emergency braking
    3. The ABS will stop working
    4. Nothing; the gauge is enough
    Show answer
    You could lose air pressure without knowing it, causing sudden emergency braking

    Without the warning you may not notice lost pressure; in a single-circuit system that can cause sudden emergency braking.

  15. In a dual air system with a failed low air warning, losing air pressure would mainly:
    1. Cause no change at all
    2. Lock all wheels immediately
    3. Increase stopping distance, with only limited braking before the spring brakes come on
    4. Turn on the brake lights
    Show answer
    Increase stopping distance, with only limited braking before the spring brakes come on

    In dual systems, stopping distance increases and only limited braking is possible before the spring brakes apply.

  16. How do you check that the spring brakes come on automatically?
    1. Drive fast and pull the knob
    2. Turn the engine off and wait 10 minutes
    3. Disconnect the air lines
    4. Keep fanning off air pressure; the parking brake valve should pop out at 20–45 psi
    Show answer
    Keep fanning off air pressure; the parking brake valve should pop out at 20–45 psi

    Continue stepping on and off the pedal; the parking brake valve (and tractor protection valve on a tractor-trailer) should pop out at 20–45 psi.

  17. On a tractor-trailer, as you fan the air down, which valves should close (pop out)?
    1. The tractor protection valve and the parking brake valve
    2. Only the safety valve
    3. The tank drain valves
    4. The front brake limiting valve
    Show answer
    The tractor protection valve and the parking brake valve

    On a tractor-trailer, both the tractor protection valve and the parking brake valve should pop out.

  18. In a dual air system at operating rpm, air pressure should build from 85 to 100 psi within:
    1. 15 seconds
    2. 45 seconds
    3. 2 minutes
    4. 3 minutes
    Show answer
    45 seconds

    Dual systems: 85 to 100 psi within 45 seconds (larger tanks may take longer; check maker specs).

  19. In older single air systems (pre-1975), the typical buildup requirement is:
    1. 85 to 100 psi in 45 seconds
    2. 20 to 45 psi in 1 minute
    3. 50 to 90 psi within 3 minutes at 600–900 rpm idle
    4. 100 to 125 psi in 10 seconds
    Show answer
    50 to 90 psi within 3 minutes at 600–900 rpm idle

    Single systems: typically 50 to 90 psi within 3 minutes with the engine idling at 600–900 rpm.

  20. If air pressure does not build up fast enough, you should:
    1. Not drive until the problem is fixed
    2. Drive in a lower gear
    3. Pump the brakes while driving
    4. Turn off the low air warning
    Show answer
    Not drive until the problem is fixed

    Slow buildup can let pressure drop too low while driving and force an emergency stop. Get it fixed first.

  21. How do you test the parking brake?
    1. Pump the pedal and watch the gauge
    2. Drive at 5 mph and pull the knob
    3. Pull on the slack adjusters
    4. Stop, set the parking brake, and gently pull against it in a low gear
    Show answer
    Stop, set the parking brake, and gently pull against it in a low gear

    Set the parking brake and gently pull against it in a low gear to check that it holds.

  22. To test the service brakes, move forward slowly at about what speed and apply the brakes firmly?
    1. 2 mph
    2. 5 mph
    3. 15 mph
    4. 25 mph
    Show answer
    5 mph

    Release the parking brake, move forward at about 5 mph, and brake firmly with the pedal.

  23. During the service brake test, what should you watch for?
    1. The ABS lamp turning green
    2. The safety valve opening
    3. Pulling to one side, unusual feel, or delayed stopping
    4. The wig wag rising
    Show answer
    Pulling to one side, unusual feel, or delayed stopping

    Note any pulling to one side, unusual feel, or delayed stopping action.

  24. For the governor-cut-out applied leakage test, the air pressure should be built up to about:
    1. 120–140 psi
    2. 55–60 psi
    3. 85–100 psi
    4. 150–175 psi
    Show answer
    120–140 psi

    Build pressure to governor cut-out (120–140 psi) before the applied leakage test.