portion flange fastener opening 84 such that fasteners 86 extending through openings 110 and 84 secure valve inner cylinder 90 in alignment within outlet portion 54 a valve piston 120 is slidably coupled between valve inner cylinder 90 and outlet portion 54 more specifically valve piston 120 is a substantially right hollow cylinder that that extends from an inlet edge 124 to an outlet edge 126 and includes an exterior surface 128 and an interior surface 130 exterior and interior surfaces 126 and 130 respectively define respective external and internal diameters d 5 and d 6 for piston 120 in the exemplary embodiment diameters d 5 and d 6 are substantially constant between piston edges 124 and 126 and both diameters d 5 and d 6 are larger than valve inner cylinder external diameter d 3 in an alternative embodiment an inlet side of piston 120 has a smaller diameter than an outlet side of piston 120 which as described in more detail below facilitates piston 120 being double acting additionally piston external diameter d 5 is slightly smaller than outlet portion bore diameter d 2 such that when piston 120 is received within outlet portion 54 piston external surface 128 is slidably coupled against outlet portion interior surface 70 between outlet portion shoulder 78 and inlet portion 52 a seal assembly 131 extends circumferentially around piston outlet edge 126 to facilitate minimizing leakage of actuation air past piston outlet edge 126 seal assembly 131 is substantially perpendicular to a centerline axis 132 extending through cylinder 120 and has an outer diameter d 7.