A cap assembly comprising: - a body having a top surface and a bottom surface, a spout engagement channel on the bottom surface thereof and an opening extending therethrough, the opening including a recessed circumferential channel and a membrane engaging flange positioned proximate the recessed circumferential channel; - a sealing membrane covering the opening and including a body attachment flange, a valve body forming a substantially cup shaped configuration extending inwardly from the attachment flange and in a depending direction toward the bottom surface of the body, the valve body including a sidewall structure with an inner surface and an outer surface, and a base wall structure spanning the inner surface of the sidewall structure, the base wall structure having a valve opening extending therethrough, and a connector region coupling the upper end (106, 114) of the valve body to the body attachment flange, the sealing membrane positioned upon the membrane engaging flange so that the body attachment flange and the membrane engaging flange are in overlying engagement; and - a retaining ring including a tab, a sealing membrane engagement surface and an inner wall structure opposite the sealing membrane engagement surface, the retaining ring positioned with the sealing membrane engagement surface in overlying engagement with the sealing membrane, whereupon extension of the tab into the recessed circumferential channel of the body releasably maintains said overlying engagement and biases the retaining ring against the sealing membrane, characterized in that, the inner surface is angled inwardly towards the bottom surface of the body so that the thickness of the sidewall structure increases away from an upper end (106, 114) thereof. - a body having a top surface and a bottom surface, a spout engagement channel on the bottom surface thereof and an opening extending therethrough, the opening including a recessed circumferential channel and a membrane engaging flange positioned proximate the recessed circumferential channel; - a sealing membrane covering the opening and including a body attachment flange, a valve body forming a substantially cup shaped configuration extending inwardly from the attachment flange and in a depending direction toward the bottom surface of the body, the valve body including a sidewall structure with an inner surface and an outer surface, and a base wall structure spanning the inner surface of the sidewall structure, the base wall structure having a valve opening extending therethrough, and a connector region coupling the upper end (106, 114) of the valve body to the body attachment flange, the sealing membrane positioned upon the membrane engaging flange so that the body attachment flange and the membrane engaging flange are in overlying engagement; and - a retaining ring including a tab, a sealing membrane engagement surface and an inner wall structure opposite the sealing membrane engagement surface, the retaining ring positioned with the sealing membrane engagement surface in overlying engagement with the sealing membrane, whereupon extension of the tab into the recessed circumferential channel of the body releasably maintains said overlying engagement and biases the retaining ring against the sealing membrane,