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Air Brakes CDL Practice Test

The Air Brakes test is required for any vehicle equipped with a full air brake system, an air-over-hydraulic brake system, or air-actuated parking brakes. It covers system components, dual air brake systems, parking brakes, brake fade, brake adjustment, leak rates, low-air warnings, emergency brakes, the application pressure gauge, and the daily air brake pre-trip inspection sequence.

Why this test exists

An air brake failure on a loaded combination vehicle can cause runaway, jackknife, or fire. The CDL Air Brakes test exists because air systems behave very differently from hydraulic brakes — drivers must understand pressure build, air loss tolerance, and stopping distance under load.

How to study this section

Memorize the seven steps of the air brake pre-trip check, the leak-down rates for single and combination vehicles, the warning signal pressure threshold, and the spring brake automatic application pressure range. Practice the order of operations — never rely on muscle memory in the booth.

62 practice questions

• Q1. Air brakes use compressed air to apply braking force. The three braking systems on most CMVs are:
• Q2. The compressor governor controls:
• Q3. Typical compressor cut-out pressure is approximately:
• Q4. The air pressure warning signal must activate before pressure drops below:
• Q5. Spring brakes typically apply automatically when air pressure falls to approximately:
• Q6. The air brake system air loss rate (with engine off, brakes released) for a single vehicle should no…
• Q7. For a combination vehicle, maximum allowed air loss with brakes released is:
• Q8. When inspecting the air brake system, you should first build pressure to:
• Q9. Brake fade is most likely on:
• Q10. The proper braking technique for a long downgrade is:
• Q11. The supply line (emergency line) on a tractor-trailer is connected by the:
• Q12. The service line (control line) is connected by the:
• Q13. The trailer hand valve (trolley valve, Johnson bar) should be used:
• Q14. A dual air brake system has:
• Q15. A leaking air tank with the pressure dropping rapidly while parked indicates:
• Q16. Brake chambers convert air pressure into:
• Q17. A slack adjuster controls:
• Q18. When checking manual slack adjuster travel, the rule of thumb is the chamber pushrod should not move…
• Q19. When draining air tanks, you should:
• Q20. The alcohol evaporator (used in some systems) helps prevent:
• Q21. The parking brake control on most newer trucks is:
• Q22. Bobtail tractors (no trailer attached) require:
• Q23. When the air brake system loses all pressure, what stops the vehicle?
• Q24. The required leak-down test before driving consists of:
• Q25. Anti-lock braking systems (ABS) on tractors and trailers are intended to:
• Q26. If the ABS warning light stays on after start-up:
• Q27. Stab braking is appropriate on:
• Q28. The tractor protection valve protects the tractor from:
• Q29. The trailer air supply valve (red knob) should be:
• Q30. The most common cause of brake fade on a long downgrade is:
• Q31. When parking on a downgrade, the front wheels should be turned:
• Q32. The proper sequence for the seven-step pre-trip air brake check ends with:
• Q33. The application pressure gauge shows:
• Q34. Rest of brake fade prevention includes:
• Q35. When you make an emergency stop in a vehicle with ABS, you should:
• Q36. The compressor is most often driven by:
• Q37. A water-saturated air brake system can lead to:
• Q38. Air brake supply pressure should generally fall between:
• Q39. A driver who hears continuous air leakage during a leak-down test should:
• Q40. The ratio of brake force from the front service brakes versus rear is balanced by:
• Q41. A 'full brake application' is performed during inspection by:
• Q42. Fanning the brakes during downhill descent will:
• Q43. The tractor protection valve closes automatically when trailer air pressure drops to:
• Q44. When uncoupling, before disconnecting the air lines you should:
• Q45. The function of the one-way check valve in the air system is to:
• Q46. A hissing sound that continues after the engine is shut off and brakes released usually indicates:
• Q47. You should never use the parking brake while:
• Q48. A safety valve on the supply tank typically vents at approximately:
• Q49. The brake system warning device may be:
• Q50. Modulating valve (treadle valve) is the:
• Q51. When testing the spring brakes during pre-trip you should expect:
• Q52. Air dryers in the system serve to:
• Q53. The S-cam in a drum brake system rotates to:
• Q54. Wedge brake actuators differ from S-cam brakes in that they:
• Q55. A glad hand seal that is cracked or missing will:
• Q56. The first sign of a low air pressure problem while driving is usually:
• Q57. When climbing a long upgrade, brake adjustment should:
• Q58. When the trailer service brake glad hand becomes disconnected, the trailer brakes will:
• Q59. When you apply the trailer hand valve, you should not use it to:
• Q60. A vehicle that has been sitting unused for several days may have:
• Q61. A driver should consider the brake system to be in unsafe condition when:
• Q62. After applying brakes during a stop, you should:

State-specific resources

Each U.S. state administers the CDL knowledge test through its own DMV. The federal questions are the same; the test scheduling, fees, and identification requirements vary. Start with your state's overview:

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