A pneumatically operated gas valve mounted on a vessel containing pressurized gas. The gas valve includes a piston positioned in a cylinder with one closed end so that the piston seats against a gas outlet to close the gas valve. A control reservoir is formed in the cylinder between the piston and the closed end of the cylinder. Upon filling vessel, some of the pressurized gas enters the control reservoir to provide a control pressure behind the piston. Actuating the control mechanism vents the control reservoir, resulting in the gas valve opening rapidly to release the pressurized gas in the vessel through an exhaust port of the gas valve.