What happens to the tractor protection valve when there is a major leak in the emergency brake line?

The correct understanding involves the operation of the tractor protection valve in response to a major leak in the emergency brake line. When such a significant leak occurs, the tractor protection valve's primary function is to protect the air supply to the trailer's braking system. It accomplishes this by closing off the air supply to the trailer.

As a result of this closure, the emergency brakes on the trailer are engaged or activated automatically. This is a critical safety feature designed to prevent the trailer from rolling or becoming detached in the event of an air supply failure, thereby ensuring the safety of the vehicle and others on the road.

The other options suggest different outcomes that would not occur in the event of a significant leak. For instance, if the valve were to reopen, this could result in a loss of air pressure to the trailer, leading to uncontrolled movement. If it remained unaffected, the emergency brakes would not engage, which would be a safety hazard. Additionally, while a warning light could provide some level of feedback, it is not the immediate or automatic response that occurs in the face of a significant brake line leak. Therefore, the mechanism of the tractor protection valve closing and thereby applying the trailer's emergency brakes is the correct function in this scenario.