what is spring brake relay valve? (air brake system)

A spring brake relay valve is a type of relay valve that applies the spring brakes much-more-quickly than would otherwise be possible. It 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. The function of this valve is to maintain the spring in the brake actuator compressed even though a system failure be sensed and to vent the air from the rear chamber of the brake actuator in an inverse ratio to the air at service pressure supplied to the service brake chamber by the operator through the conventional treadle valve.