CDL Air Brake - Commercial Driver's License Air Brakes Air Pressure and Leakage Tests Questions and Answers — Questions and Answers
Question 1: During an air brake pre-trip inspection on a combination vehicle, you perform the applied leakage test. After the initial pressure drop, what is the maximum allowable decrease in air pressure over a one-minute period?
- 2 psi
- 3 psi
- 4 psi (Correct answer)
- 5 psi
Correct answer: 4 psi
For a combination vehicle, the maximum acceptable air pressure loss during the one-minute applied leakage test (with the service brake fully applied) is 4 psi. For a single vehicle, the limit is 3 psi.
Question 2: A driver is performing the air pressure buildup test. With the engine at normal operating RPMs, the pressure must build from 85 to 100 psi within what timeframe for a dual air brake system?
- 30 seconds
- 45 seconds (Correct answer)
- 60 seconds
- 90 seconds
Correct answer: 45 seconds
In a vehicle with a dual air brake system, the air compressor should build pressure from 85 to 100 psi within 45 seconds when the engine is running at a normal operating RPM (typically 600-900 RPMs).
Question 3: You are conducting a static leakage test on a straight truck. The procedure involves fully charging the system, turning off the engine, and releasing all brakes. After the initial drop, the air pressure should not drop more than:
- 4 psi in one minute
- 3 psi in one minute
- 2 psi in one minute (Correct answer)
- 1 psi in one minute
Correct answer: 2 psi in one minute
During a static leakage test on a single vehicle (like a straight truck), the air pressure should not drop more than 2 psi in one minute after the initial stabilization. For a combination vehicle, the limit is 3 psi per minute.
Question 4: A driver is testing the low air pressure warning system. After turning the engine off and the key to the 'on' position, the driver begins to fan the brake pedal. The warning light and buzzer MUST activate at or before what pressure?
- 85 psi
- 45 psi
- 75 psi
- 60 psi (Correct answer)
Correct answer: 60 psi
According to federal regulations, the low air pressure warning devices (light and/or buzzer) must activate at or before the air pressure drops to 60 psi. Some sources may mention a range, but the critical safety threshold is not to go below 55-60 psi before the warning activates.
Question 5: While testing the automatic application of the spring brakes, a driver fans the brake pedal to lower the system's air pressure. At what approximate pressure range should the parking brake valve pop out, causing the spring brakes to engage?
- 80-100 psi
- 60-75 psi
- 20-45 psi (Correct answer)
- Below 20 psi
Correct answer: 20-45 psi
The spring brakes are designed to apply automatically as a safety measure when air pressure gets too low. The parking brake valve (and tractor protection valve on combination vehicles) should pop out when the pressure drops into the 20 to 45 psi range.
Question 6: Which of the following scenarios describes a failed air pressure buildup test, requiring immediate mechanical attention?
- The governor cuts out at 130 psi.
- The air pressure builds from 85 to 100 psi in 40 seconds.
- The low air warning buzzer activates at 65 psi.
- The air pressure takes 55 seconds to build from 85 to 100 psi. (Correct answer)
Correct answer: The air pressure takes 55 seconds to build from 85 to 100 psi.
A key standard for the air brake system is that the compressor must build air pressure from 85 to 100 psi in 45 seconds or less. Taking 55 seconds indicates a potential problem with the compressor or a leak in the system, and thus constitutes a failed test.
CDL Air Brake - Commercial Driver's License Air Brakes Air Pressure and Leakage Tests Questions and Answers
During an air brake pre-trip inspection on a combination vehicle, you perform the applied leakage test.
After the initial pressure drop, what is the maximum allowable decrease in air pressure over a one-minute period?