The hand valve of a tractor-trailer
- controls the tractor brakes but not the trailer brakes.
- should not be used for parking. ✓
- can be used to apply the tractor brakes when coupling the tractor to a trailer.
- controls the spring (emergency) brakes on the vehicle.
Answer: B. should not be used for parking.
The hand valve should not be used for parking a tractor-trailer.
Why this is the answer
The parking passage is direct: set the parking brakes and block the wheels, and never use the trailer hand valve or the tractor protection valve to hold a parked unit. The brakes passage also describes the hand valve as a way to apply trailer brakes independently, not as a parking brake. That supports the bank answer: the hand valve is not a parking device for a tractor-trailer.
Why the other answers are wrong
- A. controls the tractor brakes but not the trailer brakes. — The handbook says the trailer hand valve applies the trailer brakes independently, so it does not control only the tractor brakes.
- C. can be used to apply the tractor brakes when coupling the tractor to a trailer. — The hand valve applies trailer brakes independently, not the tractor brakes, so it is not for applying tractor brakes when coupling.
- D. controls the spring (emergency) brakes on the vehicle. — The passages treat emergency brakes separately from the hand valve, and describe the hand valve as applying trailer brakes, not spring or emergency brakes.
Remember it
Remember the hand valve as a trailer-brake control, not a parking hold: park with the parking brakes and wheel blocks.
Official handbook: Official MTO Truck Handbook, Parking