If there is a loss of pressure in the trailer supply line, you would expect the trailer brakes to be applied and
- released every 2 seconds.
- the tractor to lose all air pressure.
- the tractor brakes to remain operable. ✓
- the trailer supply valve to remain open.
Answer: C. the tractor brakes to remain operable.
With a loss of pressure in the trailer supply line, the trailer brakes apply and the tractor brakes remain operable.
Why this is the answer
The handbook says a sudden loss of air through a broken trailer supply line makes the trailer air supply valve close automatically. The tractor protection valve then blocks the passage to the trailer control line, so air is not wasted through the break. This protects the tractor air supply from dropping to an unsafe level, and a foot valve application still applies the tractor service brakes. The trailer brakes also dynamite and stay applied while pressure remains in the trailer reservoir.
Why the other answers are wrong
- A. released every 2 seconds. — Nothing in the passages describes the trailer brakes being released every two seconds; the trailer brakes stay applied as long as pressure is retained in the trailer reservoir.
- B. the tractor to lose all air pressure. — The tractor protection valve is there to protect the tractor air supply from being depleted to an unsafe level, so the tractor does not lose all its air pressure.
- D. the trailer supply valve to remain open. — The trailer air supply valve shuts off automatically when air is lost through the broken supply line, so it does not remain open.
Remember it
Think of the protection valves closing ranks when supply-line pressure disappears, so the tractor keeps its air.