CDL Air Brakes TestDual Air Brake SystemsHard
A driver performs a brake applied air loss test on a straight truck equipped with a dual air brake system. After building pressure to 125 psi, turning off the engine, and applying the service brakes firmly, the driver observes that the air pressure drops from 125 psi to 110 psi in the primary system and from 125 psi to 112 psi in the secondary system over one minute. Is this an acceptable rate of air loss?
- AYes, as long as the low air warning does not activate during the test.
- BNo, because the pressure drop in the primary system exceeds the 2 psi per minute limit.
- CYes, because the total pressure drop is less than 5 psi per minute.
- DNo, because the allowable pressure drop for a straight truck is 3 psi per minute.
Show answer & explanationAnswer & explanation
Correct answer: D. No, because the allowable pressure drop for a straight truck is 3 psi per minute.
For a straight truck (single vehicle), the maximum allowable air loss with the engine off and service brakes applied is 3 psi in one minute. The primary system dropped 15 psi (125-110) and the secondary dropped 13 psi (125-112), both significantly exceeding the 3 psi limit, indicating excessive leaks.
Why the other options are wrong
- A. While the low air warning not activating is a good sign, it doesn't negate an excessive leak rate which indicates a safety hazard needing repair.
- B. The 2 psi per minute limit applies to the initial air loss test without brakes applied for a straight truck, not the applied brake test. The limit for applied brakes is 3 psi.
- C. The allowable drop is 3 psi for a straight truck, not 5 psi for a combination vehicle. The individual system drops are 15 psi and 13 psi, far exceeding any acceptable limit.
Straight Truck Applied Air Loss Test Limit
For a straight truck, with the engine off and service brakes firmly applied, the air pressure should not drop more than 3 psi in one minute.
- Engine off, brakes applied
- Maximum 3 psi drop in 1 minute
- Indicates excessive leaks if exceeded
Memory trick: Remember 'three' for 'straight truck' when you're testing the brakes.