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CDL air brakes practice test

The air brakes test is the one that keeps the air brake restriction off your licence. It covers the parts of the system, how to inspect it, the pressure numbers you must know by heart, and how to brake safely on long grades. These sample questions are drawn from Section 5 of the FMCSA CDL manual.

Free air brakes practice questions

Ten questions from the PassRig bank. Pick an answer in your head, then show the answer to read why. The full bank, timed mock exams and AI explanations live in the app.

  1. An air brake system is really three braking systems combined. They are the:

    • Service, parking and emergency brakes
    • Drum, disc and wedge brakes
    • Foot, hand and engine brakes
    • Primary, secondary and trailer brakes
    Show answer
    Answer: A. Service, parking and emergency brakes

    The service brakes stop you when you press the pedal, the parking brakes hold the vehicle with spring pressure, and the emergency brakes use parts of both to stop you if air is lost.

    Source: FMCSA CDL Manual, Section 5.1 (The Parts of an Air Brake System)

  2. Brake lag in an air brake system is the time it takes for air to reach the brakes. At 55 mph on dry pavement this adds about how much to your stopping distance?

    • Almost nothing
    • About 32 feet
    • About 100 feet
    • About 300 feet
    Show answer
    Answer: B. About 32 feet

    Air brakes take roughly half a second to flow through the lines after you press the pedal. At 55 mph that half second is about 32 feet of travel added on top of perception, reaction and braking distance.

    Source: FMCSA CDL Manual, Section 5.3.1 (Braking Distance and Brake Lag)

  3. Brake fade is caused by:

    • Too much air pressure
    • Excessive heat from using the brakes too much
    • Low oil in the compressor
    • Driving in cold weather
    Show answer
    Answer: B. Excessive heat from using the brakes too much

    Brakes stop you by turning motion into heat. Too much heat and the drums expand and the linings lose grip, so the same pedal pressure gives less braking. Let the engine and gears hold your speed so the brakes stay cool.

    Source: FMCSA CDL Manual, Section 5.4.2 (Brake Fading or Failure)

  4. If the air compressor is belt driven, you should check the belt during your inspection for:

    • Proper tightness and condition
    • The manufacturer's name
    • Whether it is the same color as the fan belt
    • Nothing; compressor belts never need checking
    Show answer
    Answer: A. Proper tightness and condition

    A loose or worn belt can slip or break, and then the compressor stops supplying air. The compressor may also be gear driven, and it needs oil either from the engine or its own supply.

    Source: FMCSA CDL Manual, Section 5.1.1 (Air Compressor)

  5. When you release the brake pedal, what happens to the air that was applying the brakes?

    • It returns to the tanks for reuse
    • It stays in the chambers until the next stop
    • It goes into the tires
    • It is let out of the brake chambers, so tank pressure drops and the compressor must replace it
    Show answer
    Answer: D. It is let out of the brake chambers, so tank pressure drops and the compressor must replace it

    Each brake application uses up stored air. Pressing and releasing the pedal repeatedly can use air faster than the compressor replaces it, which is why fanning the brakes is dangerous.

    Source: FMCSA CDL Manual, Section 5.1.7 (The Brake Pedal)

  6. A 'wig wag' low-pressure warning device:

    • Flashes the headlights when the brakes are hot
    • Sounds a horn when the tanks are full
    • Vibrates the brake pedal when ABS is active
    • Drops a mechanical arm into your view when air pressure falls below about 60 psi
    Show answer
    Answer: D. Drops a mechanical arm into your view when air pressure falls below about 60 psi

    The low air warning may be a red light, a buzzer, or a wig wag arm that swings down in front of you. On some vehicles the arm rises out of view only when pressure is above 60 psi.

    Source: FMCSA CDL Manual, Section 5.1.9 (Low Air Pressure Warning)

  7. The modulating control valve found on some vehicles lets you:

    • Adjust the cut-in and cut-out pressures
    • Change how fast the tanks drain
    • Turn the ABS on and off
    • Apply the spring brakes gradually with a hand lever, for example if the service brakes fail
    Show answer
    Answer: D. Apply the spring brakes gradually with a hand lever, for example if the service brakes fail

    The further you move the lever, the harder the spring brakes apply. It is spring-loaded so you can feel the braking, and it can be locked in place to hold the vehicle parked.

    Source: FMCSA CDL Manual, Section 5.1.13 (Modulating Control Valves)

  8. In a dual air brake system, the primary system usually operates:

    • The brakes on the front axle
    • The trailer brakes only
    • The brakes on the rear axle or axles
    • The parking brakes only
    Show answer
    Answer: C. The brakes on the rear axle or axles

    Most newer heavy vehicles have two separate air systems sharing one compressor. The primary system runs the rear brakes and the secondary the front, so a leak in one leaves the other to stop you.

    Source: FMCSA CDL Manual, Section 5.2 (Dual Air Brake Systems)

  9. In a dual air system, pressure should build from 85 to 100 psi within:

    • 45 seconds
    • 3 minutes
    • 10 seconds
    • 5 minutes
    Show answer
    Answer: A. 45 seconds

    Run the engine at operating rpm and time the gauge. Slow build-up means a weak compressor or a leak, and you may run low on air while driving.

    Source: FMCSA CDL Manual, Section 5.3.3 (Check Rate of Air Pressure Build-up)

  10. When stab braking, you release the brakes as soon as the wheels lock. After release, the wheels may take how long to start rolling again?

    • About five seconds
    • They roll instantly
    • Up to one second
    • About half a minute
    Show answer
    Answer: C. Up to one second

    Stab braking means applying full brakes, releasing when the wheels lock, and reapplying as soon as the wheels roll. If you reapply before they are rolling, the vehicle will not straighten out.

    Source: FMCSA CDL Manual, Section 5.4.2 (Emergency Stops: Stab Braking)

What the air brakes test covers

  • Parts of an air brake system: compressor, governor, tanks, valves
  • Dual air brake systems and spring brakes
  • Pressure numbers: cut-in, cut-out, low air warning, spring brake pop-out
  • Inspecting air brakes: the applied leakage test and the governor test
  • Brake lag and total stopping distance with air brakes
  • Braking on downgrades and brake fade
  • Emergency and parking brake use

Who needs it

Anyone who wants to drive a vehicle with air brakes. Skip it and your CDL carries an L or Z restriction.

The test has 25 multiple-choice questions and the passing score is 80%, so you can miss 5. Your state DMV or DOT administers it; the federal rules are on the FMCSA CDL page. Study Section 5 of the CDL manual alongside these questions.

More CDL practice tests

Air Brakes test questions

How many questions are on the CDL air brakes test?

25 questions. You need 20 right, the 80% pass mark.

What happens if I skip the air brakes test?

Your CDL gets an air brake restriction, so you cannot drive a vehicle with full air brakes. Most trucking jobs need it, so almost every driver takes the test.

Is the air brakes test an endorsement?

No. It removes a restriction instead of adding an endorsement, but it is a separate written knowledge test with its own 80% pass mark.

Which numbers do I have to memorise for the air brakes test?

The ones the test loves: low air warning at about 60 psi, spring brakes pop out between 20 and 45 psi, governor cut-out around 125 psi, cut-in around 100 psi, and the leak-down limits in the applied leakage test.