Once the trailer supply valve closes, the tractor protection valve on the towing vehicle also closes. This prevents air loss out of the towing vehicle when there is no trailer attached. The trailer supply valve controls the tractor protection valve. When the trailer supply valve is opened to allow air to the trailer, air pressure is also directed to open the tractor protection valve and allow air to the trailer service line. When the trailer supply valve is closed, air pressure exhausts from the tractor protection valve and it closes by spring force. When the tractor protection valve is closed, air enters the valve but does not pass through it.