What is the main purpose of the trailer hand valve?
- To operate the tractor protection valve
- To regulate air pressure between the tractor and trailer
- To allow the driver to disconnect the air lines in case of emergency
- To allow the driver to independently control the amount of application air pressure to be directed to the trailer brakes ✓
Answer: D. To allow the driver to independently control the amount of application air pressure to be directed to the trailer brakes
The trailer hand valve lets the driver control the trailer brakes independently and set how much application air pressure goes to them.
Why this is the answer
The handbook says the hand valve allows independent control of the trailer brakes and lets you apply them independently of the tractor. It also explains that the two-way check valve sends the highest application pressure from either the hand valve or the foot valve to the trailer brakes. That matches the purpose described in the correct choice: the hand valve gives the driver separate control over the amount of application pressure directed to the trailer brakes. The handbook adds that the hand valve is not for parking and should not be used for normal or emergency braking.
Why the other answers are wrong
- A. To operate the tractor protection valve — The passages describe the hand valve as controlling the trailer brakes independently, not as operating the tractor protection valve.
- B. To regulate air pressure between the tractor and trailer — The trailer air supply valve is the part described as directing air pressure to the trailer, while the hand valve's job is independent trailer brake control.
- C. To allow the driver to disconnect the air lines in case of emergency — The handbook warns against using the hand valve for normal or emergency braking, and it does not describe it as a way to disconnect air lines.
Remember it
Remember: hand valve means trailer brakes only; use the foot valve for service braking.