CDL Combination Vehicles TestCombination Vehicle Air BrakesMedium
A driver is performing a brake test on a combination vehicle. After building the air pressure to the governor cut-out level, the driver turns off the engine, releases the parking brake, and fully applies the foot brake. If the air pressure drops more than 4 psi in one minute for a combination vehicle, what does this indicate?
- AThis is a normal and acceptable air loss rate for a combination vehicle.
- BThe air compressor is faulty and needs immediate repair.
- CThe low air warning system is malfunctioning.
- DThere is an excessive air leak in the brake system.
Show answer & explanationAnswer & explanation
Correct answer: D. There is an excessive air leak in the brake system.
This scenario describes the air loss rate test. For combination vehicles, with the engine off and foot brake fully applied, an air pressure drop of more than 4 psi in one minute indicates excessive leaks in the air brake system, which is a critical safety defect.
Why the other options are wrong
- A. A drop of more than 4 psi is NOT normal; it indicates a problem.
- B. A faulty air compressor would primarily affect the pressure build-up time or cut-out/cut-in pressures, not necessarily the air loss rate with the engine off.
- C. The low air warning system is tested by fanning the brakes down, not by observing air loss with the foot brake applied.
Air Loss Rate Test (Combination Vehicle)
A critical pre-trip inspection test to determine if there are excessive air leaks in the brake system of a combination vehicle by monitoring pressure drop with the foot brake applied.
- Engine off, parking brake released, foot brake fully applied.
- Pressure should not drop more than 4 psi in one minute for combination vehicles.
- Excessive drop indicates leaks that must be repaired.
Memory trick: Leaky Lines Lose Pressure Quickly.