When should the tractor protection valve and parking brake valve close?

The correct choice is that the tractor protection valve and parking brake valve close when air pressure falls to the manufacturer's specification. The primary function of the tractor protection valve is to maintain the air supply for the braking system of the tractor and trailer. It closes to prevent the loss of air pressure when it drops to a critical level, which is typically set by the manufacturer. This helps ensure that the braking system remains functional and prevents potential accidents caused by insufficient air pressure.

In addition, when air pressure is maintained above the manufacturer's minimum specifications, the brakes can be fully released and the vehicle can operate normally. The closure of the valves when air pressure drops to critical levels is a safety measure designed to protect the integrity of the braking system and maintain control of the vehicle, particularly in situations where the air compressor may be unable to maintain adequate pressure.