Commercial Driver's License (CDL) Air Brakes and Combination Vehicles Syllabus

Air Brakes and Combination Vehicles syllabus — full chapter and topic list

Expand any chapter to see its topics and sub-topics. This is the whole examinable outline for Air Brakes and Combination Vehicles in Commercial Driver's License (CDL), not a summary of it.

1. 1

   Air Brake System Fundamentals

   4 topics

   • Components: compressor, governor, reservoirs, and valves
   • Dual air brake systems and supply pressure
   • Foundation brakes, S-cam, and brake drums
   • Spring brakes and parking brake controls
2. 2

   Air Brake Operation and Inspection

   4 topics

   • Normal braking and stab braking on downgrades
   • Air loss rate, leakage, and warning devices
     • Low air pressure warning at 60 psi
     • Spring brake automatic activation
   • Testing brake lag and air buildup time
   • Draining tanks and managing brake fade on long grades
3. 3

   Combination Vehicle Driving

   4 topics

   • Rollover and rearward amplification (crack-the-whip)
   • Steering and turning with trailers
     • Off-tracking and proper lane positioning
   • Managing trailer skids and jackknifing
   • Driving empty vs. loaded combination units
4. 4

   Coupling and Uncoupling

   4 topics

   • Fifth wheel and kingpin coupling steps
   • Connecting and securing the trailer
     • Glad hands and air line connections
     • Electrical (pigtail) connection
   • Inspecting the coupling for security
   • Safe uncoupling and trailer support

Air Brakes and Combination Vehicles flashcards for Commercial Driver's License (CDL)

24 of 61 cards from the Air Brakes and Combination Vehicles deck — real questions with worked answers.

1. What three things does an air brake system use compressed air to do, and what is the basic principle?

   It uses compressed air to make the brakes work; air pressure applies force to the brake shoes/pads. Air brakes are actually three braking systems combined: the service brake, the parking brake, and the emergency brake.

2. What is the function of the air compressor in an air brake system?

   The air compressor pumps air into the air storage reservoirs (tanks) to build and maintain the system's air pressure.

3. How is the air compressor connected to and driven by the engine?

   It is connected to the engine through gears or a V-belt and may be air-cooled or cooled by the engine's cooling system; it may have its own oil supply or be lubricated by engine oil.

4. What does the air compressor governor control, and at what pressures does it typically cut out and cut in?

   The governor controls when the compressor pumps air into the tanks. It stops the compressor from pumping (cut-out) at about 125 psi and resumes pumping (cut-in) at about 100 psi.

5. What is the purpose of the air storage reservoirs (tanks) in an air brake system?

   They hold compressed air. There must be enough air capacity to allow the brakes to be used several times even if the compressor stops working.

6. Why must air tanks be drained, and what collects in them?

   Compressed air contains water and compressor oil that collect in the tanks; this contamination can harm the brake system. Tanks must be drained to remove the water and oil so they don't freeze or damage components.

7. What is the difference between manual and automatic air tank drains?

   Manually operated drain valves (a petcock or pull cable) must be opened by hand to drain the tank daily. Automatic drain valves expel water and oil on their own and may include a heating device to prevent freezing.

8. What is the purpose of the safety valve (safety relief valve) on an air brake system, and at what pressure is it set?

   Installed in the first (supply/wet) reservoir, it protects the tank and system from excessive pressure. It is usually set to open at 150 psi; if it releases air, something is wrong and a mechanic should fix it.

9. What does the foot brake valve (treadle/brake pedal) do?

   Pressing the brake pedal applies air pressure to the brakes; the harder you press, the more air pressure is applied. Releasing the pedal reduces the pressure and releases the brakes. Pressing and releasing too often can drop air pressure below the level needed to refill the tanks.

10. In a foundation brake system, what is the most common type and its main parts?

    The most common type is the S-cam drum brake. Key parts include the brake drum, brake shoes and linings, the S-cam, the slack adjuster, the push rod, and the brake chamber with its diaphragm.

11. Where is the brake drum located and what happens inside it when you brake?

    The brake drum is located at each wheel end. When you brake, the brake shoes and linings are pushed against the inside of the drum; the friction slows the vehicle and creates heat.

12. Explain how an S-cam brake applies the brakes step by step.

    Air enters the brake chamber and pushes the diaphragm and push rod out, which moves the slack adjuster, twisting the brake camshaft. This turns the S-shaped cam, which forces the brake shoes apart and presses the linings against the inside of the drum.

13. What is the function of the slack adjuster in an S-cam brake?

    The slack adjuster connects the push rod to the brake camshaft and converts the push rod's linear motion into rotation of the S-cam. It also adjusts for brake lining wear to keep brake stroke within limits.

14. What causes brake fade, and why does it happen on long downgrades?

    Brake fade results from overheating the brakes from excessive use. Heat causes the brake drums to expand and the linings to lose friction, so the brakes deliver less stopping power even as you press harder.

15. How should the brakes be used to control speed on a long, steep downgrade?

    Go slow and use the engine and transmission (low gear) as the primary means to control speed; use the brakes only as a supplement. Select a safe speed and a low gear before starting down so you rarely need the brakes.

16. Describe the proper braking technique (snub/light steady braking) for a long downgrade with the right gear selected.

    Apply the brakes just hard enough to feel a definite slowdown. When your speed has been reduced about 5 mph below your safe speed, release the brakes. When you return to your safe speed, repeat. (E.g., if safe speed is 40 mph, brake to 35, release, then re-apply at 40.)

17. What is stab braking, and when is it used?

    Stab braking is an emergency technique used when you must brake hard and the vehicle does NOT have antilock brakes. It is used to stop quickly without locking the wheels and losing control.

18. Describe how to perform stab braking step by step.

    (1) Apply the brakes fully. (2) Release the brakes when the wheels lock up. (3) As soon as the wheels start rolling again, apply full braking again. It can take up to 1 second for the wheels to start rolling after you release, so do not reapply too soon.

19. Why should you NOT use stab braking on a vehicle equipped with antilock brakes (ABS)?

    With ABS, the system automatically prevents wheel lockup, so you should apply full, steady braking and keep your foot firmly on the pedal; the ABS modulates pressure for you. Stab braking is only for vehicles without ABS.

20. What is the purpose of spring brakes, and how do they work?

    Spring brakes are powerful mechanical springs held back by air pressure. When air pressure is removed (lost or intentionally applied for parking/emergency), the springs put on the brakes. They serve as both parking brakes and emergency brakes.

21. At what air pressure do the spring (emergency) brakes typically come on automatically?

    On tractors and straight trucks, the spring brakes will come on fully when air pressure drops to a range of about 20 to 45 psi (typically 20-45 psi). A low-air warning should activate well before this.

22. How are the parking brakes applied and released in a vehicle with spring brakes?

    You apply the parking brakes by pulling out the yellow, diamond-shaped parking brake control knob, which releases air and lets the springs apply the brakes. You release them by pushing the knob in, sending air pressure to hold the springs back.

23. Why should you never push the brake pedal down (apply the service brakes) when the spring (parking) brakes are already on?

    If the spring brakes are on and you also press the brake pedal, the combined force of the springs and the air pressure could damage the brakes by applying too much force. Make sure the system has enough air before releasing the parking brakes.

24. At what air pressure must the low-air-pressure warning signal come on, per regulation?

    The low-air-pressure warning signal must come on before the air pressure in the tanks falls below 60 psi (it may activate at a higher pressure on some vehicles). It is usually a red light and often a buzzer.

See more Air Brakes and Combination Vehicles flashcards →

Planning Air Brakes and Combination Vehicles for Commercial Driver's License (CDL)

Air Brakes and Combination Vehicles (Commercial Driver's License (CDL)) FAQ