In Figure 28, air is pumped by the compressor to the supply reservoir, which is protected from over-pressurization by a safety valve. Pressurized air moves from the supply reservoir to the primary reservoir (green) and the secondary reservoir (red) through one-way check valves. At this point, the dual circuits start. Air from the primary reservoir is directed to the foot valve. Air is also directed from the secondary reservoir to the foot valve. The foot valve is divided into two sections (two foot valves in one). One section of this dual foot valve controls the primary circuit and the other section controls the secondary circuit.
When a brake application is made (Fig. 29), air is drawn from the primary reservoir (green) through the foot valve and is passed on to the relay valve, which delivers air from the primary reservoir to the rear brake chambers. At the same time, air is also drawn from the secondary reservoir (red), passes through the foot valve and is passed on to the front brake chambers
If there is an air loss in either circuit, the other circuit will continue to operate independently (Fig. 30 and Fig. 31). Unless air is lost in both circuits, the vehicle will continue to have braking ability. The primary and secondary circuits are equipped with low-pressure warning devices and pressure gauges.