CDL General Knowledge TestCombination Vehicles (if applicable)Medium
A driver is performing a pre-trip inspection on a combination vehicle. When checking the service brake air lines, what is an acceptable maximum air pressure drop for a fully charged system (engine off, brakes applied)?
- ANo more than 4 psi in one minute.
- BNo more than 2 psi in one minute.
- CNo more than 3 psi in one minute.
- DNo more than 1 psi in one minute.
Show answer & explanationAnswer & explanation
Correct answer: A. No more than 4 psi in one minute.
For a combination vehicle, with the engine off and the service brakes fully applied, the air pressure drop should not exceed 4 psi in one minute. A higher drop indicates a significant leak that needs to be addressed.
Why the other options are wrong
- B. This is the standard for a single unit (tractor only), not a combination.
- C. This is too low for a combination vehicle.
- D. This is too low; the standard allows more.
Combination Vehicle Air Brake Leakage Rate
The maximum allowable air pressure drop in a combination vehicle's air brake system, with the engine off and brakes applied, is 4 psi per minute.
- Applies to fully charged system (90-100 psi).
- Engine off, service brakes applied.
- Measures overall system integrity.
- Exceeding 4 psi indicates a significant leak.
Memory trick: Four for the floor, more means you're poor (on air).