The application line is referred to as a SERVICE LINE. In this simple system, used for an example, the driver depresses the foot valve treadle, and application air will be delivered to the tractor brake chambers and to the trailer brake chambers. When the driver releases the foot valve treadle, the application air to the trailer brake chambers must return to the foot valve to be exhausted to the atmosphere.

In this example system, there would be disadvantages:

If the trailer broke away from the tractor, the trailer would have no brakes.

If the service line parted or ruptured, the trailer brakes could not be applied, and the application air would be lost from the tractor if a brake application were made.

If the air in the main reservoirs were lost, there would be no way to apply the brakes of the tractor or the trailer.

The trailer brakes cannot be applied independently from the tractor, and there is no way to set the trailer brakes when coupling to the tractor.

The application and release of the trailer brakes would be slower than those of the tractor.

These disadvantages are overcome by the addition of the lines and valves discussed in the following pages.