The valve consists of a housing with an inner cavity containing a separating membrane (17) with an aperture (31), forming upper and lower chambers (16,17), the upper one containing a seat (23') for the membrane. A rod (23) of variable section passes through the membrane's aperture, and the housing has a port for a stopper (21) mounted on the rod. The valve housing is made in three sections, two of which (11,12) grip the periphery of the membrane, while the third is a base cap (27) which forms the pouter wall of one of the chambers (17). The aperture for the stopper is located in the first of the housing sections (11), and the stopper surface is chamber to form the surface of the other chamber (16).
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