Air Brakes knowledge test

CDL Air Brake System Parts Questions

This topic covers what each piece of an air brake system does: the compressor and governor, the supply tanks and their drain valves, the foot valve, slack adjusters and foundation brakes, the low air pressure warning, and the spring brakes that bring the vehicle to a stop when air is lost. Questions here tend to give you a symptom and ask which component is responsible, so knowing what a part is for matters more than being able to point at it.

What the test asks about air brake system parts

If you do not pass the Air Brakes knowledge test, your license gets an air brake restriction and you are not allowed to drive a commercial vehicle equipped with air brakes. That rules out most tractor-trailers and a lot of straight trucks, so for many drivers this test decides whether the CDL is actually usable. The system is also the reason air brakes are considered safer than hydraulic brakes: it is built so that losing air applies the brakes rather than releasing them.

Where drivers lose marks

  • Mixing up the three brake systems. The service brakes are what you use with the foot pedal, the parking brakes are held by spring pressure, and the emergency brakes come on automatically when air is lost.
  • Not knowing the standard pressure figures. The governor typically cuts out around 125 psi and cuts back in around 100 psi, the low air warning has to come on before pressure falls below 60 psi, and spring brakes apply somewhere between 20 and 45 psi.
  • Assuming more air pressure means more stopping power. Beyond the point where the brakes are fully applied, extra pressure does nothing, and repeated hard braking can fade the brakes through heat instead.
  • Forgetting to drain the air tanks. Water and compressor oil collect at the bottom, and in freezing weather that water can lock the system up.

How to study itLearn the air path in order: compressor, governor, tanks, foot valve, brake chambers, slack adjusters, drums or discs. Once you can trace where the air goes, questions that describe a fault usually point at one obvious component.

50 Air Brake System Parts practice questions with answers

Every question below is followed by the correct answer and an explanation of why it is right. Read through them, then take the timed Air Brakes test to see how you do without the answers in front of you.

1. What is the purpose of the air storage tanks?

  • A. To cool the engine during hard braking.
  • B. To hold compressed air so the brakes can be used multiple times even if the compressor stops working. (correct answer)
  • C. To mix compressed air with brake fluid.
  • D. To regulate the exact pressure going to the brake chambers.

Why: Air storage tanks hold enough compressed air to allow the brakes to be used several times, even if the compressor completely fails.

2. What was the purpose of the front brake limiting valve found on older vehicles (pre-1975)?

  • A. To increase front brake pressure for better stopping power.
  • B. To reduce the chance of the front wheels skidding on slippery surfaces by halving the air pressure to them. (correct answer)
  • C. To apply the parking brakes on the front axle only.
  • D. To limit the speed of the vehicle when going downhill.

Why: Older vehicles had a limiting valve to cut the air pressure to the front brakes by half to prevent skidding on slippery surfaces.

3. If the air compressor is V-belt driven, what should you check during the pre-trip inspection?

  • A. Ensure the belt has been removed for winter driving.
  • B. Check the belt's condition and tightness. (correct answer)
  • C. Check that the belt is lubricated with engine oil.
  • D. Verify the belt is painted a bright safety color.

Why: If the compressor is belt driven, you must check the condition and tension of the belt during the pre-trip inspection to ensure it will not slip or break.

4. Where are the brake drums located on the vehicle?

  • A. In the engine compartment.
  • B. Attached to the air compressor.
  • C. Bolted to the wheels, with the braking mechanism located inside them. (correct answer)
  • D. On the dashboard as part of the brake controls.

Why: Brake drums are located on each end of the vehicle's axles and are bolted to the wheels. The braking mechanism is inside the drum.

5. Which brake system uses parts of the service and parking brake systems to stop the vehicle in the event of a brake system failure?

  • A. The service brake system.
  • B. The emergency brake system. (correct answer)
  • C. The anti-lock brake system (ABS).
  • D. The secondary brake system.

Why: The emergency brake system uses parts of the service and parking brake systems to stop the vehicle if there is a primary system failure.

6. How large must the air storage tanks be in an air brake system?

  • A. Large enough to hold air for at least 50 full brake applications.
  • B. They only need to hold enough air for a single stop.
  • C. Large enough to allow the brakes to be used several times if the compressor stops working. (correct answer)
  • D. They must be exactly 50 gallons in capacity.

Why: Air tanks must be large enough to hold enough air to allow the brakes to be used several times even if the compressor stops working.

7. How are air compressors in heavy vehicles typically cooled?

  • A. By the vehicle's engine cooling system or by outside air. (correct answer)
  • B. By an independent refrigerant system.
  • C. By cold compressed air from the storage tanks.
  • D. By a dedicated electric cooling fan.

Why: Air compressors are generally cooled by the engine cooling system or by outside air as they operate.

8. If your vehicle is equipped with an alcohol evaporator, what must you do during daily cold-weather maintenance?

  • A. Check the alcohol container and fill up as necessary. (correct answer)
  • B. Drain the alcohol evaporator completely every night.
  • C. Mix the alcohol with antifreeze before filling.
  • D. Bypass the evaporator if temperatures drop below freezing.

Why: You must check the alcohol container daily in cold weather and fill it as necessary to ensure ice does not form in the system.

9. Why must you never push down the brake pedal when the spring brakes are applied?

  • A. Because it will cause the air compressor to overheat.
  • B. Because the combined forces of the springs and the air pressure could damage the brake components. (correct answer)
  • C. Because it will drain all the engine oil.
  • D. Because it will reverse the polarity of the stop light switch.

Why: If you push the brake pedal when spring brakes are applied, the combined force of the spring and the air pressure could severely damage the brakes.

10. At what air pressure must the low air pressure warning signal come on?

  • A. Before the pressure falls to 100 psi.
  • B. Before the pressure falls to 80 psi.
  • C. Before the pressure falls to 60 psi. (correct answer)
  • D. Before the pressure falls to 20 psi.

Why: The low air pressure warning signal must come on before the pressure in the tanks falls below 60 psi.

11. How do wedge brakes operate differently from S-cam brakes?

  • A. They use hydraulic fluid instead of compressed air.
  • B. The pushrod pushes a wedge directly between the ends of two brake shoes, forcing them apart. (correct answer)
  • C. They use a power screw to clamp brake pads onto a rotor.
  • D. They only act as parking brakes, not service brakes.

Why: In wedge brakes, the brake chamber pushrod pushes a wedge directly between the ends of two brake shoes, shoving them apart and against the drum.

12. What is a 'wig-wag' in an air brake system?

  • A. A device that sweeps water off the windshield using compressed air.
  • B. A mechanical arm that drops into the driver's view to warn of low air pressure. (correct answer)
  • C. A valve that fluctuates air pressure rapidly during an emergency stop.
  • D. A gauge that shows application pressure.

Why: A wig-wag is a type of mechanical low air pressure warning device that drops a mechanical arm into the driver's view when pressure falls below 60 psi.

13. What creates the friction needed to stop the vehicle in a drum brake system?

  • A. The slack adjuster pressing against the pushrod.
  • B. The brake linings (shoes) pressing against the inside of the brake drum. (correct answer)
  • C. The S-cam rubbing against the brake chamber.
  • D. The air pressure directly pushing against the tire.

Why: When you apply the brakes, the brake shoes and linings are pushed against the inside of the brake drum to create friction and slow the vehicle.

14. An air brake system is actually comprised of three separate braking systems. What are they?

  • A. Service, parking, and emergency brake systems. (correct answer)
  • B. Primary, secondary, and auxiliary brake systems.
  • C. Front, rear, and trailer brake systems.
  • D. Compression, hydraulic, and mechanical brake systems.

Why: A commercial vehicle air brake system consists of the service brake system, the parking brake system, and the emergency brake system.

15. If air pressure is lost, at what pressure range will the tractor protection valve close and the spring brakes automatically apply (pop out)?

  • A. Between 100 and 125 psi.
  • B. Between 60 and 80 psi.
  • C. Between 20 and 45 psi. (correct answer)
  • D. Between 5 and 15 psi.

Why: If air pressure drops into the range of 20 to 45 psi, the spring brakes will automatically apply, usually causing the parking brake knob to pop out.

16. What does the parking brake control knob on the dashboard look like in a newer vehicle?

  • A. A blue octagon.
  • B. A red square.
  • C. A yellow diamond. (correct answer)
  • D. A green circle.

Why: In modern vehicles, the parking brake control is a yellow, diamond-shaped push-pull control knob.

17. What can happen if you repeatedly press and release (pump) the brake pedal unnecessarily?

  • A. It builds up extra air pressure in the tanks.
  • B. It keeps the brakes cooler on long downgrades.
  • C. It can let air out of the system faster than the compressor can replace it, reducing pressure. (correct answer)
  • D. It automatically adjusts the slack adjusters.

Why: Pressing and releasing the brake pedal unnecessarily can let air out faster than the compressor can replace it, resulting in dangerously low air pressure.

18. What happens if moisture in the air lines freezes?

  • A. The air pressure will safely vent out of the safety valve.
  • B. The compressor will run backwards.
  • C. The ice can block the air lines and cause complete brake failure. (correct answer)
  • D. The brakes will lock up automatically immediately.

Why: Moisture can collect and freeze in air lines in cold weather. Ice can block the lines, preventing air from reaching the brakes and causing brake failure.

19. On large buses, what type of low air pressure warning device is required?

  • A. A visual light only.
  • B. An audible buzzer only.
  • C. A signal that you can both see and hear. (correct answer)
  • D. A wig-wag only.

Why: On large buses, it is common and often required that the low air pressure warning device must be a signal that you can both see and hear.

20. How are the moving parts of the air compressor typically lubricated?

  • A. They must be manually greased during pre-trip inspections.
  • B. They use a specialized internal synthetic oil reservoir.
  • C. They are lubricated by their own oil supply or by engine oil. (correct answer)
  • D. They are lubricated by the moisture collected in the air tanks.

Why: Air compressors can have their own oil supply, or they can be lubricated by the engine oil.

21. What is a dual parking control valve system?

  • A. A system with two brake pedals for the driver and instructor.
  • B. A system that applies both tractor and trailer parking brakes simultaneously.
  • C. A system with a separate air tank used to release the spring brakes temporarily so the vehicle can be moved in an emergency. (correct answer)
  • D. A system that controls parking brakes on dual tires only.

Why: Some vehicles have a dual parking control valve, which uses air from a separate tank to let you release the spring brakes to move a short distance in an emergency.

22. How do air-operated disc brakes work?

  • A. Air pressure turns an S-cam that spreads the brake shoes apart.
  • B. Air pressure pushes a wedge between two brake shoes.
  • C. Air pressure acts on a brake chamber and slack adjuster, engaging a power screw that clamps brake pads against a rotor. (correct answer)
  • D. Air pressure creates a vacuum that pulls the brake drum inward.

Why: In disc brakes, air pressure operates a brake chamber and slack adjuster to turn a power screw, which clamps brake pads against a rotor.

23. At what air pressure level does the air compressor governor typically 'cut-out' (stop pumping air)?

  • A. Around 100 psi
  • B. Around 125 psi (correct answer)
  • C. Around 150 psi
  • D. Around 60 psi

Why: The governor stops the compressor from pumping air (cuts out) when the air pressure rises to the cut-out level, typically around 125 psi.

24. In an S-cam brake, what happens when compressed air is allowed into the brake chamber?

  • A. The air directly presses the brake pads against the rotor.
  • B. Air pressure pushes the pushrod out, moving the slack adjuster, twisting the brake camshaft. (correct answer)
  • C. The air pulls the brake shoes away from the drum.
  • D. The air engages the parking brake spring.

Why: Air pressure pushes the pushrod out, moving the slack adjuster, which then twists the brake camshaft to engage the S-cam.

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

  • A. To control the air pressure applied to the brakes.
  • B. To pump air into the air storage tanks. (correct answer)
  • C. To release excess air pressure from the system.
  • D. To cool the compressed air before it reaches the brake chambers.

Why: The air compressor pumps air into the air storage tanks (reservoirs) to provide the compressed air necessary to operate the brake system.

26. If your vehicle is equipped with an application pressure gauge, what does it show?

  • A. The pressure of the engine oil.
  • B. How much air pressure is in the primary tank.
  • C. How much air pressure you are applying to the brakes. (correct answer)
  • D. The current pressure rating of the safety valve.

Why: An application pressure gauge shows how much air pressure you are currently applying to the service brakes when you press the brake pedal.

27. What is the function of the return spring located on the brake shoes?

  • A. To push the pushrod out of the brake chamber.
  • B. To pull the brake shoes away from the drum when the brake pedal is released. (correct answer)
  • C. To hold the parking brakes in place.
  • D. To push the S-cam back into the slack adjuster.

Why: The return spring connects the two brake shoes and pulls them away from the drum when air pressure is released, disengaging the brakes.

28. What does it mean if the safety valve releases air while you are operating the vehicle?

  • A. The system is functioning perfectly.
  • B. The brakes are applied too forcefully.
  • C. There is a malfunction, such as the governor failing to cut out, and a mechanic is needed. (correct answer)
  • D. The air tanks are completely empty.

Why: If the safety valve releases air, something is wrong (often the governor has failed to cut out). The system has too much pressure and must be fixed by a mechanic.

29. Which component controls when the air compressor will pump air into the storage tanks?

  • A. The safety valve
  • B. The treadle valve
  • C. The air compressor governor (correct answer)
  • D. The modulating control valve

Why: The air compressor governor controls when the air compressor will pump air into the air storage tanks by monitoring system pressure.

30. Which part of the brake assembly converts air pressure into mechanical force?

  • A. The brake chamber (correct answer)
  • B. The brake drum
  • C. The S-cam
  • D. The governor

Why: The brake chamber contains a diaphragm or piston that pushes the pushrod out when air enters, converting air pressure to mechanical force.

31. What is the purpose of a modulating control valve in the cab?

  • A. To control the air pressure to the windshield wipers.
  • B. To apply the spring brakes gradually in an emergency. (correct answer)
  • C. To regulate the air compressor's cut-out pressure.
  • D. To drain the air tanks automatically.

Why: A modulating control valve is used to apply the spring brakes gradually to maintain control of the vehicle if the service brakes fail.

32. What is the standard pressure setting for the safety valve in an air brake system?

  • A. 100 psi
  • B. 125 psi
  • C. 150 psi (correct answer)
  • D. 200 psi

Why: The safety valve is usually set to open at 150 psi to protect the tank and the rest of the system from too much pressure.

33. If your vehicle has automatic air tank drains, what must you remember?

  • A. They never need to be inspected or maintained.
  • B. They only drain water, so oil must still be drained manually.
  • C. They may be equipped with electric heating devices to prevent freezing in cold weather. (correct answer)
  • D. They require a special key to operate.

Why: Automatic air tank valves expel water and oil automatically, and they may be equipped with electric heating devices to prevent freezing in cold weather.

34. What component connects the brake chamber pushrod to the S-camshaft?

  • A. The brake shoe
  • B. The return spring
  • C. The slack adjuster (correct answer)
  • D. The air line

Why: The slack adjuster connects the pushrod to the camshaft, forming a lever that rotates the camshaft when the pushrod moves.

35. Which component do you press with your foot to apply the service brakes?

  • A. The modulating control valve
  • B. The brake pedal (or treadle valve) (correct answer)
  • C. The parking brake control knob
  • D. The limiting valve

Why: The brake pedal (also called a foot valve or treadle valve) is pressed to apply the service brakes during normal driving.

36. When driving under normal conditions, what holds the powerful spring brakes back (keeps them disengaged)?

  • A. A heavy mechanical latch.
  • B. The brake pedal.
  • C. Air pressure. (correct answer)
  • D. The slack adjuster.

Why: During normal driving, air pressure is supplied to the parking brake chambers, compressing the powerful springs and holding the brakes off.

37. How is the air compressor connected to the engine?

  • A. Always by a V-belt.
  • B. Always by direct gear drive.
  • C. By a dedicated electric motor.
  • D. It can be gear driven or V-belt driven. (correct answer)

Why: Air compressors are connected to the engine and can be either gear driven or driven by a V-belt.

38. What activates the stop light switch when you apply the air brakes?

  • A. A mechanical linkage connected directly to the brake pedal.
  • B. An electric switch that is operated by air pressure. (correct answer)
  • C. A hydraulic sensor in the brake fluid line.
  • D. The spring brake control valve.

Why: The air brake system uses an electric switch operated by air pressure. When you apply the brakes, the air pressure turns on the brake lights.

39. Federal regulations require all trucks, truck tractors, and buses with air brakes to have parking brakes held on by what?

  • A. Hydraulic pressure.
  • B. Air pressure from the emergency tank.
  • C. Mechanical force, typically using spring brakes. (correct answer)
  • D. A manual locking pin.

Why: Federal law requires that parking brakes be held on by mechanical force, because air pressure can eventually leak away. Spring brakes are usually used to meet this requirement.

40. Why are modern vehicles no longer equipped with a manual front brake limiting valve?

  • A. Because they were replaced by spring brakes.
  • B. Because testing showed that front wheel braking is crucial in all conditions to prevent jackknifing and maintain steering control. (correct answer)
  • C. Because front wheels no longer use drum brakes.
  • D. Because it is now controlled by the air compressor governor.

Why: Testing proved that reducing front brake pressure actually increased stopping distance and the likelihood of a crash, so manual limiting valves are no longer used.

41. What is the purpose of an alcohol evaporator in an air brake system?

  • A. To clean the brake shoes of grease and debris.
  • B. To put alcohol into the air system to reduce the risk of ice forming in valves and lines. (correct answer)
  • C. To boost the air pressure in emergency situations.
  • D. To act as a secondary fuel source for the compressor.

Why: An alcohol evaporator introduces alcohol into the air system to help reduce the risk of ice forming in air brake valves and lines during cold weather.

42. Which brake system applies the parking brakes when you pull the yellow diamond-shaped knob?

  • A. The service brake system.
  • B. The emergency brake system.
  • C. The exhaust brake system.
  • D. The parking brake system. (correct answer)

Why: The parking brake system applies the parking brakes (usually using spring brakes) when you pull the parking brake control knob.

43. What is the most common type of foundation brake found on heavy vehicles?

  • A. Wedge brakes
  • B. Disc brakes
  • C. S-cam drum brakes (correct answer)
  • D. Hydraulic drum brakes

Why: Foundation brakes are used at each wheel. The most common type is the S-cam drum brake.

44. Why must air tanks be drained regularly?

  • A. To relieve excess pressure from the system.
  • B. To prevent air from becoming stale.
  • C. To remove water and compressor oil that can freeze or damage the system. (correct answer)
  • D. To recalibrate the supply pressure gauge.

Why: Compressed air usually has some water and compressor oil in it, which can collect in the tanks. If not drained, this mixture can freeze in cold weather and cause brake failure.

45. Which brake system applies and releases the brakes when you use the brake pedal during normal driving?

  • A. The parking brake system.
  • B. The emergency brake system.
  • C. The service brake system. (correct answer)
  • D. The exhaust brake system.

Why: The service brake system is the primary system used during normal driving to slow down and stop the vehicle using the brake pedal.

46. What is a dual air brake system?

  • A. A system that uses both air and hydraulic fluid.
  • B. A vehicle with an air compressor on the tractor and another on the trailer.
  • C. A system that has two separate air brake systems (primary and secondary) that use a single set of brake controls. (correct answer)
  • D. A braking system specifically designed for dual-tire axles.

Why: A dual air brake system has two separate air brake systems (primary and secondary), each with its own tanks and lines, but operated by a single set of controls.

47. If your vehicle is equipped with manual air tank drains, how often should they be drained?

  • A. Once a week.
  • B. At the end of each day of driving. (correct answer)
  • C. During every oil change.
  • D. Only when the low air pressure warning activates.

Why: Manual air tank drains should be opened at the end of each day of driving to ensure all moisture and oil are removed.

48. When the air tank pressure falls, at approximately what level will the governor 'cut-in' and allow the compressor to start pumping again?

  • A. 60 psi
  • B. 80 psi
  • C. 100 psi (correct answer)
  • D. 125 psi

Why: When the air tank pressure falls to the cut-in pressure (around 100 psi), the governor allows the compressor to start pumping again.

49. What is the function of the slack adjuster in the air brake system?

  • A. To regulate the air compressor cut-in pressure.
  • B. To adjust the space between the brake shoes and the drum to account for wear. (correct answer)
  • C. To drain excess moisture from the air lines.
  • D. To limit the brake pressure applied to the front wheels.

Why: The slack adjuster is the mechanical link between the brake chamber pushrod and the camshaft, and it adjusts the slack caused by brake shoe wear.

50. What does the supply pressure gauge indicate to the driver?

  • A. How much pressure is currently being applied to the foot valve.
  • B. How much air pressure is available in the air storage tanks. (correct answer)
  • C. The pressure level at which the spring brakes will pop out.
  • D. The current operating temperature of the air compressor.

Why: The supply pressure gauge shows how much air pressure you have in the air tanks, indicating the amount of air available for braking.

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