CDL Air Brake - Commercial Driver's License Air Brakes Pre-Trip Air Brake Inspection Questions and Answers — Questions and Answers
Question 1: During a static air leakage test on a combination vehicle, what is the maximum permissible pressure drop in one minute after the initial drop?
- 2 psi
- 3 psi (Correct answer)
- 4 psi
- 5 psi
Correct answer: 3 psi
For a combination vehicle, with the engine off and the brakes released, the air pressure should not drop more than 3 psi in one minute after the initial pressure drop. For a single vehicle, the limit is 2 psi.
Question 2: A driver is performing a pre-trip inspection and tests the low air pressure warning device. At what pressure should the warning light and buzzer typically activate?
- Below 80 psi
- At or above 100 psi
- At or above 60 psi (Correct answer)
- Between 20 and 40 psi
Correct answer: At or above 60 psi
The low air pressure warning light and buzzer must come on at a pressure of 60 psi or higher (or at the manufacturer's specified level, which is typically at or above 60 psi) to alert the driver of low air pressure in the tanks.
Question 3: When testing the air compressor governor, what are the typical cut-in and cut-out pressures?
- Cut-in at 80 psi, cut-out at 110 psi
- Cut-in at 100 psi, cut-out at 125 psi (Correct answer)
- Cut-in at 125 psi, cut-out at 150 psi
- Cut-in at 60 psi, cut-out at 90 psi
Correct answer: Cut-in at 100 psi, cut-out at 125 psi
The air compressor governor is typically set to cut-in (start pumping air) at around 100 psi and cut-out (stop pumping air) at around 125 psi. This ensures the air tanks stay within the proper operating range.
Question 4: A driver applies and holds the foot brake for one minute with the engine off. For a single vehicle, what is the maximum allowable air pressure loss during this applied leakage test?
- 4 psi
- 2 psi
- 5 psi
- 3 psi (Correct answer)
Correct answer: 3 psi
During the applied air pressure test for a single vehicle (like a straight truck or bus), the pressure drop should not exceed 3 psi in one minute after the initial drop. For a combination vehicle, this limit is 4 psi.
Question 5: Which of the following steps is performed to test that the spring brakes will engage automatically?
- Pump the brake pedal to reduce air pressure until the parking brake valve pops out. (Correct answer)
- Gently pull against the parking brake in a low gear.
- Build the air pressure to the governor cut-out.
- Apply the service brake firmly while moving at 5 mph.
Correct answer: Pump the brake pedal to reduce air pressure until the parking brake valve pops out.
To check the automatic application of the spring brakes, the driver must fan the brake pedal to lower the air pressure. The parking brake valve (and tractor protection valve on combination vehicles) should pop out between 20 and 45 psi, engaging the spring brakes.
Question 6: What is the primary purpose of the tractor protection valve, which is checked during a pre-trip inspection?
- To prevent the trailer brakes from locking up during hard braking.
- To regulate the air pressure sent to the service brakes.
- To prevent total air loss from the tractor if the trailer breaks away or has a major leak. (Correct answer)
- To manually apply the trailer brakes independently of the tractor brakes.
Correct answer: To prevent total air loss from the tractor if the trailer breaks away or has a major leak.
The tractor protection valve is a critical safety device that closes automatically to prevent air from escaping the tractor's brake system in the event of a trailer breakaway or a severe air leak in the trailer's air lines.
CDL Air Brake - Commercial Driver's License Air Brakes Pre-Trip Air Brake Inspection Questions and Answers
During a static air leakage test on a combination vehicle, what is the maximum permissible pressure drop in one minute after the initial drop?