What issue does a broken return spring in the tractor protection valve cause?
- It will prevent the tractor from coupling with the trailer
- It will result in air loss during normal brake applications if operating the tractor without a trailer ✓
- It will result in continuous air loss if operating the tractor without a trailer
- It will cause the spring brakes or relay emergency brakes to come on
Answer: B. It will result in air loss during normal brake applications if operating the tractor without a trailer
A broken return spring can let air escape from the tractor during normal brake applications when no trailer is attached.
Why this is the answer
The handbook explains that the tractor protection valve closes by spring force once the trailer supply valve is closed, and that this keeps air from leaving the towing vehicle when no trailer is connected. If the return spring is broken, the valve may not close as it should. The handbook's test applies the service brakes with the trailer service line disconnected and the trailer supply valve closed; air exhausting then means the valve is defective. That fits air loss during brake applications on a tractor without a trailer.
Why the other answers are wrong
- A. It will prevent the tractor from coupling with the trailer — The passages discuss air loss from the towing vehicle, not any effect on coupling the tractor to the trailer.
- C. It will result in continuous air loss if operating the tractor without a trailer — The passages describe air escaping when the valve is tested with the service brakes applied, not as a constant leak.
- D. It will cause the spring brakes or relay emergency brakes to come on — The passages connect spring brake application to the trailer supply valve and trailer tests, not to a broken return spring in the tractor protection valve.
Remember it
Picture the valve as a door held shut by a spring: if the spring lets go, air slips past when you apply the brakes with no trailer.