A pneumatically operated gas valve includes a piston positioned in a cylinder with one closed end so that the piston may seat against a gas outlet to close the gas valve. A control reservoir may be formed in the cylinder between the piston and the closed end of the cylinder. Means for filling the control reservoir with gas to a control pressure may be provided so that the control pressure acting against the piston may close the gas valve. A release valve may be opened to allow the gas in the control reservoir to escape through an exhaust port to open the gas valve.