The diagram above illustrates the air flow action during a brake application being made with the foot valve (6). The application air has applied the tractor brakes and the trailer brakes together. As previously explained, the two way check valve (22) has shifted, and application air is being directed through the tractor protection valve (23) to the service line.

This control pressure moves through the service line to act on the relay emergency valve (25). This control pressure will cause the relay emergency valve to direct reservoir air from the trailer tank to the trailer brake chambers (26). This will be of the same P.S.I. as the control pressure, which was the P.S.I. of the application air by the foot valve. In such a system, brake lag time has been minimized.

Release of the foot valve treadle stops the flow of application air. The relay portions of the valves return to their original positions, stopping the flow of air pressure. The exhausting ports of the valves exhaust the air pressure from the brake chambers, releasing the brakes.

In this system, the brakes of both units can be released quickly.