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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.

  • Vehículo comercial
  • Estados Unidos
  • 80 preguntas
  • Revisado el 10 de octubre de 2026

Respuesta rápida

El examen oficial tiene 25 preguntas. Para aprobar: Varies by state; usually 25 questions, with 20 correct needed (80%). Lo organiza: Your state driver licensing agency, under federal rules set by FMCSA (49 CFR Part 383). Practica aquí gratis con 80 preguntas, cada una con la respuesta y una explicación. Contrastado con fuentes oficiales el 10 de octubre de 2026.

Lee antes de practicarEmpieza por CDL Air Brakes Study Guide: notas en lenguaje sencillo, diagramas y las cifras que hay que recordar.

  1. Pregunta 1 de 25

    What are the three braking systems on a vehicle with air brakes?
    Ver respuesta

    Respuesta: 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. Pregunta 2 de 25

    How is the air compressor driven?
    12345678
    Ver respuesta

    Respuesta: B. By the engine, through gears or a V-belt

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

  3. Pregunta 3 de 25

    Your air compressor has its own oil supply. What should you do before driving?
    Ver respuesta

    Respuesta: A. Check the compressor's oil level

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

  4. Pregunta 4 de 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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    Ver respuesta

    Respuesta: C. The governor

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

  5. Pregunta 5 de 25

    At about what pressure does the governor stop the compressor from pumping (cut-out)?
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    Ver respuesta

    Respuesta: B. About 125 psi

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

  6. Pregunta 6 de 25

    At about what pressure does the governor let the compressor start pumping again (cut-in)?
    Ver respuesta

    Respuesta: B. About 100 psi

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

  7. Pregunta 7 de 25

    Why must air tanks be drained?
    12345678
    Ver respuesta

    Respuesta: 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. Pregunta 8 de 25

    If your air tanks have manual drain valves, how often should you drain them?
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    Ver respuesta

    Respuesta: D. At the end of each driving day

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

  9. Pregunta 9 de 25

    What does an alcohol evaporator do?
    Ver respuesta

    Respuesta: 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. Pregunta 10 de 25

    Your vehicle has an alcohol evaporator and manual tank drains. Do you still need to drain the tanks each day?
    Ver respuesta

    Respuesta: C. Yes

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

  11. Pregunta 11 de 25

    Where is the air system's safety valve installed?
    Ver respuesta

    Respuesta: B. In the first tank the compressor fills

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

  12. Pregunta 12 de 25

    At what pressure is the safety valve usually set to open?
    Ver respuesta

    Respuesta: C. 150 psi

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

  13. Pregunta 13 de 25

    The safety valve releases air while you drive. What does this mean?
    Ver respuesta

    Respuesta: 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. Pregunta 14 de 25

    What is another name for the brake pedal in an air brake system?
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    Ver respuesta

    Respuesta: C. The foot valve or treadle valve

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

  15. Pregunta 15 de 25

    Why should you not pump the brake pedal when it is not needed?
    Ver respuesta

    Respuesta: 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. Pregunta 16 de 25

    What is the most common type of foundation brake on heavy vehicles?
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    Ver respuesta

    Respuesta: B. The S-cam drum brake

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

  17. Pregunta 17 de 25

    In an S-cam brake, what forces the brake shoes against the drum?
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    Ver respuesta

    Respuesta: 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. Pregunta 18 de 25

    In an S-cam brake, what pulls the brake shoes away from the drum when you release the pedal?
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    Ver respuesta

    Respuesta: A. A return spring

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

  19. Pregunta 19 de 25

    How do wedge brakes work?
    Ver respuesta

    Respuesta: 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. Pregunta 20 de 25

    How do air disc brakes work?
    Ver respuesta

    Respuesta: 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. Pregunta 21 de 25

    What does the supply pressure gauge show?
    Ver respuesta

    Respuesta: 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. Pregunta 22 de 25

    What does the application pressure gauge show?
    Ver respuesta

    Respuesta: 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. Pregunta 23 de 25

    You must keep increasing application pressure to hold the same speed on a downgrade. What does this mean?
    Ver respuesta

    Respuesta: 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. Pregunta 24 de 25

    Before tank pressure falls below what level must the low air pressure warning come on?
    Ver respuesta

    Respuesta: 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. Pregunta 25 de 25

    On large buses, the low air pressure warning often comes on at what pressure?
    Ver respuesta

    Respuesta: D. 80 to 85 psi

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

Qué debes saber antes del examen

  • 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.

Preguntas frecuentes

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.

Fuentes

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