the diaphragm is confined by the peripheral edge of the frustroconical cage portion 24 and the mating cylindrically shaped retainer 30 consequently any gases that permeate the nitrile material of the diaphragm may become trapped along the fabric as mentioned under increasing temperature and pressure the gases could cause a separation between the fabric and the adjacent diaphragm material and eventually lead to blistering of the diaphragm as will be seen the present invention enables any entrained gases to move along the fabric and be released from the diaphragm at selected areas reference to fig2 shows a cross sectional view of one form of the invention wherein the frustroconical cage 24 of baffle 23 has a plurality of notches 24a formed in its peripheral edge 24b reference to fig3 and 4 discloses the construction of one form of diaphragm 32 which incudes a woven polyester fabric 32a as described above embedded such that substantially equal thicknesses of nitrile material are on each side of the fabric the diaphragm may be molded with a top annular ridge 32c for assisting in compression of the diaphragm against the cage retainer 30 to prevent leakage between the diaphragm and the retainer notches 24a formed in edge 24b enable portions of the ends 32b of fabric 32a of the diaphragm to be exposed to downstream pressure and allows any.