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A valve comprises a housing (1) having an inlet (4) and an outlet (5), and a pressure sensing port (8). A piston (6) slides in a part (3) of the housing (1) in response to a difference between a first fluid pressure at the pressure sensing port on the one side of the piston, and a second fluid pressure at the inlet and/or outlet on the other side of the piston (6). A valve member (7) carried by the piston is operable thereby to close the inlet (4) when said second fluid pressure is less than a value sufficiently greater than said first fluid pressure. The valve member (7) is movable with respect to the piston (6) to facilitate closing of the inlet (4), in response to a fluid flow from the housing (1) to the inlet (4), when the piston is not acting to close the inlet.