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CDL Air Brakes Practice Test 2026

You must pass the air brakes knowledge test to drive a commercial vehicle with air brakes. These questions cover how the system works, the pressures and leakage limits you must know, the air brake inspection, and how to brake safely.

  • Commercial vehicle
  • United States
  • 80 questions
  • Checked October 10, 2026

Quick answer

The CDL air brakes knowledge test has 25 questions. Pass mark: Varies by state; usually 25 questions, with 20 correct needed (80%). It is run by Your state driver licensing agency, under federal rules set by FMCSA (49 CFR Part 383). Practise here for free with 80 questions, each with the answer and an explanation. Checked against official sources on October 10, 2026.

Read before you practiseStart with the CDL Air Brakes Study Guide: plain-language notes, diagrams and the numbers to remember.

  1. Question 1 of 25

    What are the three braking systems on a vehicle with air brakes?
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    Answer: B. Service brakes, parking brakes and emergency brakes

    Air brake vehicles have a service brake system, a parking brake system and an emergency brake system.

  2. Question 2 of 25

    How is the air compressor driven?
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    Answer: B. By the engine, through gears or a V-belt

    The compressor is connected to the engine through gears or a V-belt.

  3. Question 3 of 25

    Your air compressor has its own oil supply. What should you do before driving?
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    Answer: A. Check the compressor's oil level

    If the compressor has its own oil supply, check its oil level before driving.

  4. Question 4 of 25

    This diagram shows the parts of an air brake system. Which part controls when the compressor pumps air into the tanks?
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    Answer: C. The governor

    The governor tells the compressor when to stop and start pumping air.

  5. Question 5 of 25

    At about what pressure does the governor stop the compressor from pumping (cut-out)?
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    Answer: B. About 125 psi

    The governor's cut-out level is about 125 psi. Check your manufacturer's figure.

  6. Question 6 of 25

    At about what pressure does the governor let the compressor start pumping again (cut-in)?
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    Answer: B. About 100 psi

    The compressor starts pumping again when pressure drops to about 100 psi.

  7. Question 7 of 25

    Why must air tanks be drained?
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    Answer: B. Water and oil collect and can freeze, causing brake failure

    Water and oil build up in the tanks. In cold weather, water can freeze and make the brakes fail.

  8. Question 8 of 25

    If your air tanks have manual drain valves, how often should you drain them?
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    Answer: D. At the end of each driving day

    Manual tank drains must be drained at the end of each driving day.

  9. Question 9 of 25

    What does an alcohol evaporator do?
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    Answer: A. Adds alcohol to the air to cut ice in valves in cold weather

    An alcohol evaporator helps prevent ice in the valves and other parts during cold weather. Check and fill it daily in cold weather.

  10. Question 10 of 25

    Your vehicle has an alcohol evaporator and manual tank drains. Do you still need to drain the tanks each day?
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    Answer: C. Yes

    Daily draining is still needed unless the system has automatic drains.

  11. Question 11 of 25

    Where is the air system's safety valve installed?
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    Answer: B. In the first tank the compressor fills

    The safety valve sits in the first tank the air compressor pumps air to.

  12. Question 12 of 25

    At what pressure is the safety valve usually set to open?
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    Answer: C. 150 psi

    The safety valve protects the tanks from too much pressure. It usually opens at 150 psi.

  13. Question 13 of 25

    The safety valve releases air while you drive. What does this mean?
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    Answer: B. Something is wrong; have a mechanic fix it

    If the safety valve lets air out, there is a problem that a mechanic should repair.

  14. Question 14 of 25

    What is another name for the brake pedal in an air brake system?
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    Answer: C. The foot valve or treadle valve

    The brake pedal is also called the foot valve or treadle valve.

  15. Question 15 of 25

    Why should you not pump the brake pedal when it is not needed?
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    Answer: A. It can let out air faster than the compressor refills it

    Each time you release the pedal, air is let out. Pumping can lower the pressure until the brakes won't work.

  16. Question 16 of 25

    What is the most common type of foundation brake on heavy vehicles?
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    Answer: B. The S-cam drum brake

    S-cam drum brakes are the most common. Wedge and disc brakes are less common.

  17. Question 17 of 25

    In an S-cam brake, what forces the brake shoes against the drum?
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    Answer: D. The S-shaped cam, turned by the camshaft

    Air pushes the brake chamber rod, which turns the slack adjuster and camshaft. The S-cam pushes the shoes against the drum.

  18. Question 18 of 25

    In an S-cam brake, what pulls the brake shoes away from the drum when you release the pedal?
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    Answer: A. A return spring

    When the brakes are released, a spring pulls the shoes away from the drum.

  19. Question 19 of 25

    How do wedge brakes work?
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    Answer: A. A push rod drives a wedge between two brake shoes

    In a wedge brake, the brake chamber push rod pushes a wedge between the shoes, forcing them against the drum.

  20. Question 20 of 25

    How do air disc brakes work?
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    Answer: A. Air turns a power screw that clamps the rotor between pads

    In air disc brakes, air turns a power screw that clamps the disc (rotor) between the caliper pads.

  21. Question 21 of 25

    What does the supply pressure gauge show?
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    Answer: C. How much air pressure is in the tanks

    Supply pressure gauges show the air pressure in the tanks. Dual systems have a gauge for each half or one gauge with two needles.

  22. Question 22 of 25

    What does the application pressure gauge show?
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    Answer: B. How much air pressure is going to the brakes

    The application gauge shows how much pressure goes to the brakes. Not all vehicles have one.

  23. Question 23 of 25

    You must keep increasing application pressure to hold the same speed on a downgrade. What does this mean?
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    Answer: D. The brakes are starting to fade

    Needing more and more pressure to hold your speed means the brakes are fading. Slow down and shift to a lower gear.

  24. Question 24 of 25

    Before tank pressure falls below what level must the low air pressure warning come on?
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    Answer: D. 60 psi

    The low air warning must come on before pressure drops below 60 psi (or half the governor cut-out on older vehicles).

  25. Question 25 of 25

    On large buses, the low air pressure warning often comes on at what pressure?
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    Answer: D. 80 to 85 psi

    Large buses commonly have low pressure warnings that come on at 80 to 85 psi.

What to know before the test

  • Federal rules set the passing score for every CDL knowledge test: you must answer at least 80% of the questions correctly.
  • The air brakes test has 25 questions in states such as Washington and Utah, so you need 20 correct answers.
  • If you fail the air brakes knowledge test or take your skills test in a vehicle without air brakes, your CDL is restricted from any commercial vehicle with air brakes.
  • If you take your skills test in a vehicle with air-over-hydraulic brakes, you are restricted from vehicles with full air brakes.
  • The low air pressure warning must come on before pressure drops below 60 psi, and the governor usually cuts out at about 125 psi and cuts in at about 100 psi.

Frequently asked questions

Do I need the air brakes test?

Yes, if you will drive a commercial vehicle with air brakes. Without it, your CDL will have an air brake restriction.

How many questions are on the CDL air brakes test?

Each state sets the number. Washington and Utah both use 25 questions, so you need 20 correct to reach the 80% pass mark.

What is the allowed air leakage rate?

With the engine off and brakes released, a single vehicle should lose less than 2 psi per minute and a combination less than 3 psi. With the brakes applied, the limits are 3 psi and 4 psi per minute.

At what pressure do the spring brakes come on?

Spring brakes come on fully when air pressure drops to about 20 to 45 psi, typically 20 to 30 psi. Stop safely as soon as the low air warning comes on.

Sources

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