This valve (Fig. 35) allows air to be directed to one delivery pipe from either of two sources. A two-way check valve allows the source applying the higher pressure to shift the shuttle so that the higher pressure will be directed to the delivery port.
With this piping arrangement, the vehicle can have a failure in either circuit without the spring brakes applying auto matically. Unless air is lost in both circuits, the spring brakes will not apply.
In Figure 35 the primary circuit has a higher air pressure than the secondary circuit. The shuttle has blocked off the secondary port and the spring brakes are held off by primary air pressure. This also works in the opposite way.