The hand valve of a tractor-trailer
- controls the tractor brakes but not the trailer brakes.
- should not be used for parking. ✓
- controls the spring (emergency) brakes on the vehicle.
- can be used to apply the tractor brakes when coupling the tractor to a trailer.
Answer: B. should not be used for parking.
The hand valve should not be used for parking.
Why this is the answer
The handbook describes the hand valve as a way to apply the trailer brakes by themselves once the trailer system is connected. It then gives a direct warning against using that valve for parking. For parking, it points you to the tractor parking brakes, closing the trailer supply valve, and blocking the trailer wheels. It also says the foot valve is for service braking. That makes the statement that the hand valve should not be used for parking the one that matches the handbook.
Why the other answers are wrong
- A. controls the tractor brakes but not the trailer brakes. — The passage says the hand valve gives independent control of the trailer brakes, not the tractor brakes, so this reverses what the valve does.
- C. controls the spring (emergency) brakes on the vehicle. — The handbook ties the hand valve to trailer brake application, not to the spring or emergency brakes, so this describes a different control.
- D. can be used to apply the tractor brakes when coupling the tractor to a trailer. — The hand valve applies the trailer brakes independently; the passage does not say it applies the tractor brakes for coupling.
Remember it
Park with the tractor parking brakes and blocked wheels, not the hand valve.