If the service line is ruptured or disconnected, no action will take place until a brake application is made.

In this diagram, the service line has ruptured and the driver has made a brake application with the foot valve (6).

The application air is directed to the control line through the tractor protection valve (23). Rupture of the service line will result in the escape of air pressure, causing a loss of pressure in the tractor system. This pressure drop will cause the tractor protection system to close off, exhausting the supply line to the trailer. This will cause the trailer brakes to go into an emergency application.

The trailer brakes may be released by opening the trailer supply valve (24) to recharge the trailer system. A further application of the foot valve or hand valve would again trigger an emergency application.

NOTE: Depending on the type of tractor protection system used, the air loss from the tractor will stop immediately or it will blow down to between 45 and 20 P.S.I. and then shut off.