Which valves make up the tractor protection system?
- The trailer supply valve and the hand valve
- The foot valve and the tractor protection valve
- The trailer supply valve and the tractor protection valve ✓
- The trailer supply valve and the foot valve
Answer: C. The trailer supply valve and the tractor protection valve
The tractor protection system is made up of the trailer supply valve and the tractor protection valve.
Why this is the answer
The handbook says the trailer supply valve controls the tractor protection valve. Opening the supply valve sends air to open the protection valve; closing the supply valve lets air exhaust from the protection valve, which then closes by spring force. That linked operation is why these two valves are the pair named in the answer. The passage also says the protection valve prevents air loss from the towing vehicle when no trailer is attached.
Why the other answers are wrong
- A. The trailer supply valve and the hand valve — The hand valve applies the trailer service brakes independently; the linked pair here is the trailer supply valve and the tractor protection valve.
- B. The foot valve and the tractor protection valve — The foot valve is not the control named in the passage; the trailer supply valve is what opens and closes the tractor protection valve.
- D. The trailer supply valve and the foot valve — The passage ties the tractor protection valve to the trailer supply valve, not to the foot valve.
Remember it
Picture the supply valve as the switch and the protection valve as the door it opens and closes.