This valve is essentially another dash-mounted control valve (Fig. 37). It has two functions:
- It controls the tractor protection valve. The tractor protection valve will not operate if the trailer-supply valve is closed.
- It serves as a link between the tractor and the trailer parking-brake systems by supplying air to the trailer reservoirs, through the supply line.
Air is supplied to the trailer-supply valve by a double-check valve that is connected to both the primary and secondary circuits. The double-check valve only takes air from the highest pressure circuit, which prevents loss of air from a failed circuit.
This valve (usually a red octagonal button) is mounted in the cab of the vehicle, easily accessible to the driver. The driver opens the valve by pushing or pulling the button, depending on the type used.
Opening the valve permits main reservoir pressure to flow through the valve. This pressure is piped to the tractor protection valve and the supply-line glad hand. The spring-loaded valve is held in the open position when sufficient air pressure is reached. If pressure drops to between 20 psi (140 kPa), some valves will shut automatically by spring pressure, opening the exhaust port. On some vehicles the button may not pop out, however the spring brakes will apply. Always ensure the spring brakes have been fully applied. The driver can close the valve manually to uncover the exhaust port
Note: The trailer-supply valve has also been referred to as the emergency valve.