What happens when the trailer breaks away, pulling apart both air lines?

When the trailer breaks away and both air lines are pulled apart, the tractor protection valve closes. This is a crucial safety feature designed to prevent air from escaping the system when a disconnect occurs. By closing, the valve ensures that the air supply to the tractor's brakes remains intact, allowing the driver to maintain control of the vehicle.

If the air lines are severed, the loss of air pressure can trigger the emergency braking system of the trailer, which is designed to engage the trailer brakes automatically to prevent it from rolling away. However, the key function of the tractor protection valve is to isolate the tractor from the trailer's air supply and maintain the necessary braking ability for the remaining vehicle. This is vital for safe operation in the event of a trailer separation.