Williams Controls WM318-G Brake Ratio Relay Valve

The Williams Controls WM318-G Ratio Relay Valve is a 3-way, pilot pressure-operated pneumatic relay valve designed for heavy-duty commercial truck, trailer, and forestry air systems. Featuring a fast 1.5 PSI crack pressure and an adjustable output-to-input pressure ratio, the WM318-G precisely modulates brake line pressure to balance braking forces between tractor and trailer units. Built with standard 1/4″ NPT ports and a rugged die-cast housing for maximum reliability in extreme operating environments.

Williams Controls WM318-G Brake Ratio Relay Valve

Ensure balanced, responsive, and safe braking performance across your heavy equipment fleet with the Williams Controls WM318-G Pneumatic Ratio Relay Valve. Designed primarily for vehicular air brake systems and heavy-duty tractor-trailer setups, the WM318 series valve delivers delivery line pressure in direct proportion to the amount of pilot control pressure applied.

Engineered with dual internal compensating diaphragms, the valve activates quickly with a low 1.5 PSI crack pressure (against 100 PSI main supply). Its adjustable ratio mechanism (ranging from 1.3:1 down to 0.8:1) allows technicians to fine-tune trailer brake engagement relative to the tractor, eliminating trailer push-and-pull during braking and dramatically reducing premature brake pad and shoe wear.

Whether used as a ratio relay valve, fast brake actuator, or tractor protection component, the WM318-G is built to handle operating pressures up to 150 PSI across tough temperature extremes from -20°F to 200°F.

Technical Specifications