Abstract: The invention provides a valve (100) comprising, a housing (61) having ant enclosed internal cavity (66), an aperture (68) formed in a wall of the housing providing communication with the enclosed internal cavity, a valve scat (72) surrounding the aperture, an expandable valve member (40) mounted within the housing the expandable valve member having a hollow interior surrounded by a side wall of the expandable valve member, the side wall and hollow interior extending through a portion of the internal cavity clear of the housing where it terminates in an end wall (50), and a port (67) communicating with the hollow interior, wherein the construction and mounting of the expandable valve member and housing re such that the internal cavity of the housing may be evacuated to provide a region of sub-atmospheric pressure around the expandable valve member whereby the application of air pressure greater than sub-atmospheric pressure through the port can act to expand the expandable valve member to bring the end wall (