CDL General Knowledge TestCombination Vehicles (if applicable)Medium
What is the primary purpose of the 'trailer hand valve' (trolley valve) on a combination vehicle?
- ATo adjust the air pressure in the trailer's suspension system.
- BTo release the trailer's parking brakes during coupling.
- CTo apply the tractor's service brakes independently.
- DTo control the trailer's service brakes independently of the tractor's.
Show answer & explanationAnswer & explanation
Correct answer: D. To control the trailer's service brakes independently of the tractor's.
The trailer hand valve allows the driver to apply only the trailer's service brakes. This is useful for testing trailer brakes, preventing jackknifing on slippery surfaces, or when backing up to control the trailer's movement.
Why the other options are wrong
- A. Trailer suspension air pressure is controlled by separate valves, not the hand valve.
- B. The trailer's parking brakes are typically released by pushing in the trailer air supply knob, not with the hand valve.
- C. The foot pedal controls the tractor's service brakes, not the hand valve.
Trailer Hand Valve
A control valve in the cab of a combination vehicle that allows the driver to apply the trailer's service brakes independently of the tractor's brakes.
- Also known as 'trolley valve'.
- Used for testing trailer brakes and controlling trailer movement.
- Improper use can cause jackknifing.
Memory trick: Hand for trailer, foot for all.