This 301-4 Altitude Pilot Valve from Singer Valve is spring and diaphragm operated, and controls the water level in a reservoir by sensing the hydrostatic head. When the hydrostatic head equals the spring force, the pilot connects the main valve inlet to the main valve bonnet, closing the valve. When the hydrostatic pressure decreases slightly, the port connection is closed by the inner valve. When the hydrostatic pressure is reduced even further, the pilot connects the main valve bonnet to the exhaust to drain the air, opening the main valve. The valve body, seat and stem are made of stainless steel and encased in a ductile iron housing with a protected fusion-bonded coating. The pilot has a rolling diaphragm stem seal to eliminate friction when the pilot actuates, prolonging the life of the pilot. There is no need to remove the pilot from the valve to service. 604/594-5404; www.singervalve.com.