Abstract: A gas flow bellows valve of low profile, low purge volume and high gas flow includes an externally pressurized bellows surrounding a stepped valve stem. A self-centering toroidal guide seals in line contact with the main valve body and a bonnet with the valve stem being translatable therein. A valve actuator includes a valve piston connected to the stem and positioned between a pair of recesses in an actuator base and an actuator removable cap. A flat wave spring is positioned in one or the other of the recesses to alternatively bias the piston and stem to a valve normally closed or a valve normally open condition. Alternatively the spring may be removed entirely and the valve can function as a double-acting valve controlled by air pressure ports in the actuator base extending to one recess and in the actuator cap extending to the other recess.