CDL General Knowledge TestDriving SafelyHard
You are driving a commercial vehicle equipped with air brakes. You notice that the air pressure gauge drops from 125 psi to 100 psi after holding the foot brake for one minute. This occurs after the initial air loss when the brakes are first applied. What does this indicate?
- AThere is an excessive air leak in the brake system.
- BThe air loss rate is within acceptable limits for a single vehicle.
- CThe air compressor is functioning correctly to maintain pressure.
- DThe governor cut-in pressure is set too low.
Show answer & explanationAnswer & explanation
Correct answer: A. There is an excessive air leak in the brake system.
For a single vehicle, the air loss with the foot brake applied (Applied Air Loss Test) should not exceed 3 psi in one minute after the initial drop. A drop of 25 psi (125-100) indicates a significant and dangerous air leak.
Why the other options are wrong
- B. The acceptable applied air loss is 3 psi or less for a single vehicle; 25 psi is excessive.
- C. A 25 psi drop in one minute indicates a serious leak, not correct compressor function.
- D. The governor cut-in pressure relates to when the compressor restarts, not the rate of air loss with applied brakes.
Applied Air Loss Test
A test performed on an air brake system to check for leaks by applying the foot brake and observing the pressure drop over a specified time.
- Engine off, parking brake off, foot brake applied.
- After initial drop, pressure should not drop more than 3 psi in one minute for a single vehicle.
- For combination vehicles, not more than 4 psi in one minute.
- Excessive loss indicates dangerous leaks.
Memory trick: Foot down, pressure drops, over 3 psi, trouble crops.