The trailer supply valve is usually a push/pull type valve near the driver that supplies air to the trailer’s brake system through the supply line. This valve normally has a red, eight-sided knob that the driver pushes in to open and supply air to the trailer and pulls out to close or when no trailer is attached.
Failing to close the trailer supply valve when there is no trailer attached allows a large amount of air to escape from the towing vehicle’s air brake system. To prevent all of the air from being lost, the trailer supply valve is designed to close automatically. If the valve is open and air escapes or if the trailer breaks loose from the towing vehicle, the valve closes automatically when air pressure in the trailer supply line drops to between 138 and 311 kPa (20 and 45 psi).
Because there is a large amount of air flowing out of the trailer supply line, the pressure in the line will be considerably lower than the pressure in the air tanks. This means that in most cases the valve will close automatically well before the pressure displayed on the air pressure gauges reads between 138 and 311 kPa (20 and 45 psi).
In an emergency when the trailer service brakes fail, the trailer spring brakes can be applied by using the trailer supply valve.