CDL Combination Vehicles TestInspecting Combination VehiclesEasy
A driver is performing a pre-trip inspection of a combination vehicle. They notice that the air pressure gauge for the trailer's primary air tank is dropping rapidly after the engine is shut off and the brakes are applied. What is the most likely cause of this issue?
- AInsufficient engine RPM during air system build-up.
- BA clogged air dryer filter.
- CAn improperly adjusted slack adjuster on the tractor.
- DA leak in the trailer's air brake system.
Show answer & explanationAnswer & explanation
Correct answer: D. A leak in the trailer's air brake system.
A rapid drop in air pressure after the engine is off and brakes are applied indicates a leak in the air brake system. The primary air tank supplies air for the service brakes, and a leak would cause pressure to fall.
Why the other options are wrong
- A. Insufficient engine RPM would slow air system build-up, but not cause a rapid drop once pressure is built and the engine is off.
- B. A clogged air dryer filter would affect air system build-up or introduce moisture, not cause a rapid pressure drop with brakes applied.
- C. An improperly adjusted slack adjuster affects brake effectiveness, not air pressure loss in the primary tank.
Air Brake System Leak Detection
To detect air leaks, build system pressure to maximum, shut off the engine, apply the service brake, and monitor the air pressure gauges for drops.
- Pressure should not drop more than 3 psi in one minute for single vehicles.
- Pressure should not drop more than 4 psi in one minute for combination vehicles.
- Applying the service brake can reveal leaks in brake chambers or lines.
Memory trick: Pressure drops? Check the pipes and the seals!