Used on some vehicles with dual air systems. This device provides a means of using the force of the spring brakes to assist in stopping a vehicle in an event of a failure in the rear (primary) air system.
Fills the trailer air tanks (reservoir) and applies and releases the trailer service brakes. The relay emergency valve also applies the trailer emergency brakes in the event of a trailer breakaway or normal disconnect.
Knowledge Test Question: In the event of a trailer break away, the relay emergency valve is required to apply the trailer service brakes for a minimum of 15 minutes.