Air Brakes CDL Practice Test

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Air brakes trips up more CDL applicants than almost any other section of the knowledge test. Our $15 Class A course includes a full air brakes unit covering system nomenclature, low air pressure procedures, and pre-trip checks.

Ready for the real requirement? Class A CDL Theory theory training is $15, reported to the FMCSA registry instantly.

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  1. An air brake system on a truck tractor is actually three separate braking systems sharing components. Which three systems make up a vehicle's air brake system?
  2. Why are trucks, truck tractors, and buses required to be equipped with spring brakes (or an equivalent mechanical device) for parking and emergency braking?
  3. Why should a driver never push the brake pedal down hard while the vehicle's spring brakes are already applied?
  4. What is the function of the governor in an air brake system's air supply?
  5. During the official pre-trip Air Gauge and Governor Cutoff Check, the air pressure should build and the compressor should cut out within approximately what range?
  6. Where is the safety (relief) valve located in an air brake system, and at approximately what pressure is it normally set to open?
  7. Why is it important to drain the air tanks (manually or via automatic drain valves) regularly, especially at the end of each day's driving?
  8. In the most common type of foundation brake, the S-cam drum brake, what happens when the driver presses the brake pedal?
  9. How do wedge brakes differ from S-cam brakes in how they apply the brake shoes?
  10. On a vehicle equipped with an application pressure gauge, what does it mean if you notice you need increasing application pressure to hold the same speed down a grade?
  11. Why is unnecessarily pressing and releasing the brake pedal (treadle valve) a risky habit in a vehicle with air brakes?
  12. In a dual air brake system, which axle brakes does the 'primary' circuit typically operate, and which does the 'secondary' circuit typically operate?
  13. Before driving a vehicle equipped with a dual air brake system, how much minimum air pressure should you allow the compressor to build in BOTH the primary and secondary systems?
  14. While driving a vehicle with a dual air brake system, the low air pressure warning light and buzzer activate, meaning pressure in one of the two systems has dropped below 55 psi. What should you do?
  15. Under federal regulations, antilock braking systems (ABS) are required on truck tractors with air brakes built on or after a specific date, and on other air-brake vehicles (trucks, buses, trailers, and converter dollies) built on or after a later date. What are these two mandatory equipment dates?
  16. A driver tells a new CDL student that ABS lets you drive faster and follow other vehicles more closely because it will stop the truck quicker. Why is this advice wrong?
  17. You are doing a pre-trip inspection on a tractor-trailer and notice the trailer's ABS malfunction lamp stays on after the startup bulb check. What does this indicate, and what should you do?
  18. Which of the following correctly describes how you should brake a tractor-trailer equipped with ABS on both the tractor and the trailer during normal and emergency driving?
  19. Why is it important to drain the air tanks on an air brake system, and what can happen if you don't?
  20. An air tank without an automatic drain has a manual drain valve. How is it typically operated, and how often must the driver use it?
  21. What is the purpose of an alcohol evaporator on an air brake system, and does it eliminate the need for daily air tank drainage?
  22. At what point must the low air pressure warning signal (light and/or buzzer) activate on a vehicle with air brakes?
  23. A vehicle's mechanical 'wig-wag' low air pressure warning drops its arm into view. What triggers this, and how does it reset?
  24. The low air pressure warning activates while you are driving a loaded straight truck with air brakes. What should you do?
  25. On large buses, it is common for the low air pressure warning devices to signal at a higher pressure than the standard below-55-psi threshold used on most air-brake vehicles. What is that typical range?
  26. What is the purpose of the tractor protection valve on a combination vehicle, and when does it close automatically?
  27. If the emergency (supply) line between a tractor and trailer is cut, broken, or disconnected during a crash, what happens to the trailer's brakes, and how is the emergency line typically color-coded at the glad hand connection?
  28. At what air pressure range does the trailer air supply control (the red, octagonal, 'push to release' knob) pop out on its own, closing the tractor protection valve automatically?
  29. During the Air Gauge and Governor Cutoff Check portion of the official pre-trip air brake inspection, wheels are chocked and the engine is started. Within what range should air pressure build and then stop rising (the governor cutoff point)?
  30. During the Air Leakage Rate Test on a combination vehicle (tractor-trailer), the engine is off, the key is in the 'on'/'battery charge' position, the parking brake and tractor protection valve are released, and the foot brake is fully applied and held for one minute. What is the maximum allowable air pressure loss during that minute?
  31. During the Air Warning Device Test, you continue fanning off air pressure by applying and releasing the foot brake without restarting the engine. By what air pressure must the low-air warning devices (buzzer, light, or flag/wig-wag) activate?
  32. Continuing to fan off air pressure during the Emergency Brake(s) Test, at what air pressure must the parking/emergency brake valve 'pop out' (close), and how must an air-operated trailer's protection valve compare in timing?
  33. When checking S-cam brake slack adjusters during a walk-around inspection (vehicle parked on level ground, wheels chocked, parking brakes released), you pull hard on each reachable slack adjuster. What movement at the point where the pushrod attaches indicates the slack adjuster likely needs adjustment?
  34. All vehicles built since 1994 use automatic slack adjusters, which should self-adjust during normal full brake applications. If you find that an automatic slack adjuster needs manual adjustment to bring it back into spec, what does this most likely indicate?
  35. During the Step 7 Final Air Brake Check (with the engine running at operating RPM), how quickly should air pressure build in a dual air brake system, and between what two pressure points?
  36. During a brake component check, what crack length in a brake drum or disc is considered a defect serious enough to be flagged under out-of-service criteria?
  37. What is 'brake lag' in the context of air brakes, and roughly how long does it take?
  38. What are the four components that make up total stopping distance for a commercial motor vehicle with air brakes?
  39. In the AAMVA manual's worked example (Figure 5.6) for a vehicle with air brakes traveling 55 mph on dry pavement with good traction and an average driver, approximately how many feet does brake lag distance add, and what is the approximate total stopping distance?
  40. When making a normal stop in a vehicle with a manual transmission and air brakes, what is the correct technique regarding the clutch?
  41. What defines the 'controlled braking' method of making an emergency stop with air brakes?
  42. What is the correct procedure for the 'stab braking' emergency stop method?
  43. After releasing the brakes during stab braking because the wheels locked up, approximately how long can it take for the wheels to start rolling again, and why does reapplying too soon matter?
  44. What is the primary cause of brake fade or brake failure on a long or steep downgrade?
  45. Why does brake fade tend to get progressively worse the longer a driver keeps riding the brakes on a downgrade?
  46. A combination vehicle's trailer brakes are badly out of adjustment while the tractor brakes are properly adjusted. Why is this a safety concern on a long downgrade?
  47. What is the correct braking technique for descending a long, steep downgrade?
  48. You have just descended a long, steep grade and your brakes are very hot. You need to park the vehicle for a few minutes. What should you do?
  49. In freezing temperatures, your brakes are very wet. Why should you avoid setting the parking brake in this situation?
  50. Under normal conditions, when the brakes are not very hot or very wet and frozen, when should a driver use the parking brake?
  51. Why should you never leave a parked commercial motor vehicle unattended without applying the parking brakes or chocking the wheels?
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