CDL Combination Vehicles TestCombination Vehicle Air BrakesMedium
When driving a combination vehicle, how does a 'trailer hand valve' (sometimes called a trolley valve) differ in its application of brakes compared to the foot brake pedal?
- ABoth the hand valve and foot brake apply all brakes simultaneously, but the hand valve is for emergency use only.
- BThe hand valve applies only the tractor brakes, while the foot brake applies only the trailer brakes.
- CThe hand valve applies the trailer spring brakes, while the foot brake applies the service brakes.
- DThe hand valve applies only the trailer service brakes, while the foot brake applies both tractor and trailer service brakes.
Show answer & explanationAnswer & explanation
Correct answer: D. The hand valve applies only the trailer service brakes, while the foot brake applies both tractor and trailer service brakes.
The trailer hand valve (trolley valve) is designed to apply only the service brakes on the trailer(s). This is useful for testing or to prevent a jackknife by gently applying trailer brakes. The foot brake pedal, however, applies the service brakes on both the tractor and the trailer(s) simultaneously.
Why the other options are wrong
- A. The hand valve does not apply all brakes simultaneously, and its use is not limited to emergencies; it can be used for controlled trailer braking.
- B. This is incorrect; the hand valve is for trailer brakes, and the foot brake applies all brakes.
- C. The hand valve applies service brakes, not spring brakes. Spring brakes are applied by pulling out the red emergency knob or by loss of air pressure.
Trailer Hand Valve (Trolley Valve)
A separate control valve in the cab of a combination vehicle that allows the driver to apply only the service brakes on the trailer(s) independently of the tractor's brakes. Used for testing or to prevent jackknifes.
- Applies only trailer service brakes.
- Does NOT apply tractor brakes.
- Useful for preventing jackknifes or testing trailer brakes.
- Should generally not be used for routine stopping.
Memory trick: Foot For Full, Hand For Half.