Abstract: An electrochemical cell has a wheel shaped, molded plastic seal member of a unitary construction which includes a central cylindrical hub from which depend in outward radial sequence, an annular pressure vent, a boss, a downward projecting skirt, an annular flexure means for providing radial movement and an outer rim terminating at its periphery in a cylindrical wall. The pressure vent includes a thin membrane ring depending from the hub which joins the hub to an annular diaphragm. A hinge ring joins the periphery of the diaphragm to the boss. The seal member is designed so that the membrane ring fails primarily in shear at excessive cell pressure which makes the vent more reliable and easier to fabricate.