Abstract: An assembly is provided for use with a trailer including a plurality of wheels, normally engaged air brakes coupled to the wheels, and an air supply line coupled to the air brakes for supplying air to release the air brakes. The assembly includes a valve for selectively venting the air supply line to the atmosphere to prevent the release of the air brakes from their normally engaged position. The valve is coupled to the air supply line and is adjustable between a first position and a second position. In the first position, the valve seals the air supply line to permit the air brakes to be released when air is supplied to the air supply line. In the second position, the valve vents the air supply line to the atmosphere. When the valve is in its second position and air is supplied to the air supply line, the air is vented to the atmosphere. Therefore, in the second position, the valve means prevents the air brakes from being released when air is being supplied to the air supply line.