The trailer air supply valve is controlled by a red button on the dashboard. If supply line pressure drops to 20 – 45 psi (perhaps because the trailer separated from the tractor), the valve will close (and the red button will pop out) to prevent any more compressed air from entering the supply line. With air gone from the supply line, the trailer emergency brakes (spring brakes) will activate to stop the trailer. The trailer air supply valve works together with the tractor protection valve. Both the supply and control lines pass through the tractor protection valve (it has intake and output ports for each line). It too will close, preventing any more compressed air from leaving the tractor through either of these lines, so that the tractor’s service brakes can still work