The diagram below illustrates that the main reservoir air is piped from the main reservoir lines to the trailer supply valve (24). The tractor protection valve (23) is fed by two lines, one from the trailer supply valve (24) and one from the two-way check valve (22). Leading off from the tractor protection valve are two lines, each with a glad hand coupler. These two lines are referred to as the SERVICE LINE and the SUPPLY LINE.

In the diagram, the upper line is the service line and the lower line is the supply line.

To understand the function of the trailer supply valve and the tractor protection valve in the system, it is important to understand how they operate.