relay valve; however, the fact that the foot-valve's delivery-port outputs are still merged enables the trailer's service brakes to still be controlled even if there is failure within one braking circuit of the tractor. Spring brake relay valve A spring brake relay valve works on the same principle as the service brake relay valve, although it has the opposite effect. This type of relay valve responds to a major drop in pressure at its control-port by opening its exhaust port, which causes the air from each spring brake chamber under its control to remotely exhaust, thus applying the spring brakes much-more-quickly than would otherwise be possible if the air were required to discharge via the yellow parking-control valve on the dashboard. In a dual-circuit air brake system, air from both the primary and secondary reservoirs is fed into the supply-port of the parking-control valve, as well as the supply-port of this relay valve; it is merged via yet another two-way check valve. The delivery-port output of the parking-control valve connects to the control-port of this relay valve; this enables the spring brakes to be controlled via this valve. The merged air from the parking-control valve prevents air-loss from only one braking circuit from causing the spring brakes to automatically apply. This gives the driver more control, and prevents the vehicle from grinding to a halt in an unsafe location. However, with this increased control, comes increased responsibility on the part of the driver: If air is lost from the primary circuit alone, the spring brakes must be manually applied by the driver via the parking-control valve; otherwise, the front service brakes may not be enough to stop the vehicle safely in an emergency—especially if the vehicle is heavily loaded, and/or traveling at a high-speed. In fact, the driver's failure to manually apply the spring brakes in this situation could lead to catastrophic failure of the front brakes due to overheating, since it could cause the front service brakes to exceed their design-limit for energy absorption. QR1C air valve speeds up the process, with anti compounding, meaning trailer and service brakes will function 1 second between each other. The relay valve's function is analogous to the transistor used in electronic circuits. Testing Relay Valves Relay valves are tested for durability before use through a seat test with air. In pressure at 80 psig or more, a 2-inch or smaller relay valve