CDL Airbrakes Practice Test — Questions and Answers
Question 1: What is the normal operating pressure range for a commercial vehicle air brake system?
- 100–125 psi (Correct answer)
- 60–80 psi
- 140–160 psi
- 75–95 psi
Correct answer: 100–125 psi
The normal operating pressure range for a commercial vehicle air brake system is 100–125 psi. The compressor cuts out (stops pumping) around 125 psi and cuts back in around 100 psi. If pressure drops below 60 psi, the low air pressure warning activates.
Question 2: If the low air pressure warning activates while driving, what should you do?
- Stop safely as quickly as possible (Correct answer)
- Pump the brakes to build pressure
- Increase speed to reach a service station faster
- Downshift and continue to the nearest exit
Correct answer: Stop safely as quickly as possible
When the low air pressure warning (typically a buzzer and/or light) activates at around 60 psi, you should stop safely as quickly as possible. At pressures below 45 psi, spring brakes will automatically apply. Continuing to drive risks complete brake failure.
Question 3: What does the air pressure governor control in an air brake system?
- When the air compressor pumps air into the reservoir (Correct answer)
- The amount of braking force applied to each wheel
- The release of air from the brake chambers
- The pressure in the brake lines during application
Correct answer: When the air compressor pumps air into the reservoir
The governor controls when the air compressor pumps air. It cuts out the compressor (stops pumping) when pressure reaches the cut-out level (around 125 psi) and cuts in (starts pumping again) when pressure drops to the cut-in level (around 100 psi), maintaining the correct operating pressure range.
Question 4: What is the purpose of the air dryer in an air brake system?
- To remove moisture and oil from compressed air before it enters the tanks (Correct answer)
- To cool the compressed air to prevent overheating
- To increase air pressure above compressor output
- To regulate brake application force
Correct answer: To remove moisture and oil from compressed air before it enters the tanks
The air dryer removes moisture and oil from compressed air before it enters the air tanks. Moisture in the system can cause corrosion, freeze brake lines in cold weather, and contaminate brake components. Properly dried air extends component life and prevents winter brake freeze-ups.
Question 5: During a pre-trip inspection of air brakes, you apply the parking brake and release the service brake. Air pressure should not drop more than ____ in one minute.
- 2 psi (Correct answer)
- 5 psi
- 10 psi
- 1 psi
Correct answer: 2 psi
For a static leakage test with brakes released (single vehicle), air pressure should not drop more than 2 psi in one minute. With brakes applied, the drop should not exceed 3 psi per minute for a single vehicle or 4 psi for a combination vehicle. Excessive leakage indicates a system fault.
Question 6: What are spring brakes, and when do they apply?
- Mechanically-applied brakes that activate automatically when air pressure falls too low (Correct answer)
- Air-powered brakes that engage at high speeds
- Supplemental brakes on the trailer only
- Electric brakes that back up the air system
Correct answer: Mechanically-applied brakes that activate automatically when air pressure falls too low
Spring brakes use heavy springs inside the brake chambers to apply the brakes mechanically. They function as both parking brakes and emergency brakes. Spring brakes automatically apply when air pressure drops below approximately 45 psi, preventing a runaway vehicle if the air system fails.
CDL Airbrakes Practice Test
What is the normal operating pressure range for a commercial vehicle air brake system?