What would happen to the tractor protection valve if the air supply drops when driving?

If the air supply drops while driving, the tractor protection valve closes automatically. The tractor protection valve is a safety device in the air brake system of a tractor-trailer combination vehicle. It is designed to protect the tractor's air supply in case of a significant air loss or failure in the trailer's air brake system.

When the tractor protection valve detects a drop in the air supply, such as in the event of a trailer separation or a severe air leak in the trailer, it will automatically close to isolate the tractor's air supply. By closing the valve, it prevents further loss of air from the tractor's system and maintains the remaining air pressure for the operation of the tractor's own brakes.

This safety feature helps ensure that the tractor remains in control and can safely stop or maneuver in situations where there is a loss of air supply from the trailer.