If there is a loss of pressure in the trailer supply line, you would expect the trailer brakes to be applied and
- the tractor to lose all air pressure.
- released every 2 seconds.
- the trailer supply valve to remain open.
- the tractor brakes to remain operable. ✓
Answer: D. the tractor brakes to remain operable.
When trailer supply-line pressure is lost, the trailer spring brakes apply automatically and the tractor brakes remain operable.
Why this is the answer
The handbook says that when pressure in the trailer supply line falls below 414 kPa (60 psi), the trailer spring brakes apply by themselves. It also explains that the tractor protection valve keeps an air-loss problem in the trailer from draining air from the towing vehicle, so the tractor's brakes stay usable. That combination is why the expected result is automatic trailer braking while the tractor brakes remain operable.
Why the other answers are wrong
- A. the tractor to lose all air pressure. — The tractor does not lose all its air, because the tractor protection valve keeps a trailer air-loss problem from draining air from the towing vehicle.
- B. released every 2 seconds. — The trailer brakes are not released every 2 seconds; the passages say the trailer spring brakes apply automatically when supply-line pressure drops.
- C. the trailer supply valve to remain open. — The trailer supply valve is not described as staying open in this situation; the protective action is the tractor protection valve closing to preserve tractor air.
Remember it
Picture the tractor protection valve as a gate that shuts when the trailer side loses air, keeping the tractor's air supply for its own brakes.