A housing is threaded to the container and includes a valve stem having a flexible valve disc member. In the housing are upper and lower adjacent valve seats which mate with the disc member to form closed valves therewith. When in contact with the upper seat the valve can be opened and closed by the pressure differential between the container and the ambient. When manually placed in contact with the lower seat the disc member snaps in place by flexing over the upper seat into a locked closed valve state with the lower seat and does not open in response to pressure differentials between the container and the ambient.