CDL General Knowledge TestCombination Vehicles (if applicable)Medium
A driver is performing a pre-trip inspection on the air brake system of a combination vehicle. After ensuring the air tanks are full and the engine is off, the driver applies the service brake and holds it. The air pressure gauge drops from 120 psi to 118 psi in one minute. What does this indicate?
- AA major leak in the service brake system requiring immediate repair.
- BA minor leak that should be monitored but does not require immediate action.
- CA normal and acceptable air pressure drop for a combination vehicle.
- DA faulty air pressure gauge, as no drop should occur.
Show answer & explanationAnswer & explanation
Correct answer: C. A normal and acceptable air pressure drop for a combination vehicle.
For a combination vehicle, the air brake system should not lose more than 4 psi in one minute with the engine off and the service brake applied. A 2 psi drop is within the acceptable limits.
Why the other options are wrong
- A. A major leak would be indicated by a drop exceeding 4 psi.
- B. This is not a leak requiring monitoring; it's within normal operating parameters.
- D. Some pressure drop is normal due to the system's nature; a faulty gauge would show erratic readings or no drop at all.
Combination Vehicle Air Brake Leakage Rate
The maximum allowable air pressure drop in a combination vehicle's air brake system during a service brake application test, indicating system integrity.
- Engine off, air tanks full.
- Service brake applied and held.
- Maximum allowable drop: 4 psi in one minute.
Memory trick: Pressure drops, but not too fast, or your brake's past its last.