Ensure proper vapor recovery every time by using the Air Interlock with Limit Switch. With the limit switch, bottom loading cannot begin until a vapor recovery hose is connected to the vapor check valve. The switch does its job by breaking a connection in the secondary over-fill prevention system, thus preventing the rack monitor from giving a green light to the transport. Once a vapor line is connected, loading can begin. The coupler first depresses the plunger button of the interlock, which then locks the trailer brake system and completes the connection to the rack monitor. The vapor hose must be disconnected before the brakes will release, allowing the truck to be driven away.