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

In Figure 46, the service line has ruptured and the driver has made a brake application with the foot valve.

Application air is directed to the control line through the tractor protection valve. Rupture of the service line will result in the escape of air pressure, if the brake application is held long enough to cause enough loss of pressure in the tractor system. This pressure drop causes 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.