CDL Combination Vehicles TestInspecting Combination VehiclesHard
A driver is performing a pre-trip inspection and notices that the trailer's emergency brake (parking brake) will not release, even with adequate air pressure built up in the system and the parking brake knob pushed in. Which of the following is a possible cause?
- AExcessive play in the steering linkage.
- BA seized or rusted slack adjuster on a brake chamber.
- CA broken or disconnected emergency air line to the trailer.
- DA faulty service brake relay valve.
Show answer & explanationAnswer & explanation
Correct answer: C. A broken or disconnected emergency air line to the trailer.
The trailer's emergency (spring) brakes are held off by air pressure supplied through the emergency air line from the tractor. If this line is broken or disconnected, air cannot reach the trailer's spring brake chambers to release them, even if the tractor's parking brake knob is pushed in and system pressure is adequate.
Why the other options are wrong
- A. Excessive play in the steering linkage affects steering control and is unrelated to air brake release.
- B. A seized slack adjuster would prevent the service brake from activating or releasing fully, but the spring brake mechanism is separate and relies on air pressure for release.
- D. A faulty service brake relay valve would affect the application of service brakes, not the release of spring brakes.
Trailer Spring Brake Release Failure
Failure of trailer spring brakes to release, even with air pressure and parking brake knob pushed in, often indicates an issue with the emergency (supply) air line or the trailer's emergency relay valve.
- Emergency line supplies air to release spring brakes.
- Spring brakes apply automatically if this air pressure is lost.
- A break in the line prevents air from reaching the brake chambers.
Memory trick: No air, no go; emergency line, you know!