26b the tapered groove for the o ring 80 is vented at 88 through sleeve 78 to the low pressure in plenum 86 to prevent displacement of that o ring from its groove it will be recognized that the valve member 70 controls passage of gas from the inlet plenum to the exhaust plenum the valve member 70 is sealed relative to the plug 84 by a flexible rolling seal member 90 the outer periphery of which comprises a bead recessed into plug 84 and clamped by a ring 92 secured to the plug the inner edge of the seal member 90 is clamped between the end of sleeve 78 and a washer 94 by a nut 96 on the threaded portion 76 of the valve stem 74 the clamp ring 92 plug 84 and valve seat member 60 are retained in bore 50 by a cover 100 fixed to boss 48 as by screws 102 the valve stem 74 extends through an opening in the cover 100 and is connected at its outer end to a piston generally indicated at 104 defined between the cover 100 and the rolling seal member 90 is a balancing chamber 110 that is connected via passages 112 and 114 to the inlet port 22b.