Embodiments of the invention provide a normally open valve, a normally closed valve, and a combination valve. The valves include a valve body with an inlet coupled to a fluid supply line, an outlet providing a service flow of fluid, a vent venting to atmosphere, a lower piston, a lower diaphragm, and a mechanical valve actuator. The mechanical valve actuator includes an upper piston, an upper diaphragm, a pilot piston with a bore and a plunger, and a spring. The spring is positioned either inside the pilot piston for the normally open valve or inside the upper piston for the normally closed valve. The valves are actuated based on a control fluid entering a system pressure port and exceeding a pressure set point in order to either open or close the valve.