of recess 66 as well as urging seal 72 outwardly with respect thereto and thus ensuring a fluid tight seal between peripheral surface 84 and flange 82 in addition to pressure in the cavity biasing the valve into sealing engagement movement of the valve into sealing relation will also be aided by the static pressure created by initial gas leakage past the sealing flange 84 which static pressure will be lower than the pressure in the cavity seals 72 and 74 may be easily fabricated from any suitable and preferably somewhat resilient low friction material such as for example teflon u00ae which is a polytetrafluoroethylene plastic material a low friction material is preferred as it will aid in minimizing the resistance to movement of valving ring 50 although other suitable materials may be utilized fig7 illustrates another form of seal which may be used in place of seals 72 and 74 seal 134 comprises a relatively stiff center portion 136 which is generally flat in cross section and includes an annular outwardly projecting sealing flange portion 138 in this embodiment the diameter of recess 140 is substantially greater than recess 66 and seal 134 is secured therein by means of a suitable flexible annular diaphragm 142 or o ring which is secured at its radial inner surface to seal member 134 and at its radial outer.