The tractor protection valve is typically located behind the cab. Both the supply (emergency) and service (control) air lines pass through the tractor protection valve on their way to the trailer. (The valve has intake and output ports for each line.) In the event of a severe loss of air pressure in the supply line (perhaps because the trailer separated from the tractor), the tractor protection valve will close to prevent compressed air from leaving the tractor, so the tractor service brakes will still be able to work. It will also let the air out of the trailer supply line, which will activate the trailer emergency brakes to stop the trailer.