Figure 43 illustrates air flow during a brake application made with the foot valve. Application air has applied the tractor and trailer brakes together. As previously explained, the two-way check valve has shifted and application air is being directed through the tractor protection valve to the service line.

Control pressure moves through the service line to act on the relay emergency valve. Control pressure causes the relay emergency valve to direct reservoir air from the trailer tank to the trailer-brake chambers. Trailer-brake application pressure is the same as control pressure, which is the pressure of application air by the foot valve. In this system, brake lag is minimized.

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

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