WILLIAMS CONTROLSn WM674 SERIES DUAL FUNCTION PARKING BRAKE CONTROL DESCRIPTION The WM674 is a dual function valve used to control spring brakes under normal and emergency conditions. Each WM674 unit consists of two manually operated three-way valves housed in a common body. When pressure at its inlet exceeds 45 PSI (310 kPa), the first valve (square button) can be manually operated to control the parking brakes. From the first valve, air flows to the second valve (round button) which is spring-loaded so supply pressure can flow through it to release the spring brakes and charge the emergency supply tank. If supply pressure falls between 20-35 PSI, the first valve automatically closes and exhausts downstream pressure to cause emergency application of the spring brakes. An integral check valve in the second valve protects pressure in the emergency supply tank. The second valve can be applied to allow air to flow from the tank so the spring brakes so --- the vehicle can be moved. Since the second valve’s function is for temporary emergency use, the round button must be manually held down. As soon as the button is released, pressure at the outlet is exhausted to atmosphere and the spring brakes are reapplied. 119930 REV C 5/90 COPYRIGHT 1990 BYWILLIAMS CONTROLS, INC.