Abstract: A servo-controlled valve is described, in which a circular flexible diaphragm is inserted in a cylindrical chamber wherein the pressure can be varied in order to move the center part of the diaphragm towards and away from a valve seat. The diaphragm is loosely inserted in the chamber and has a peripheral flange fitting to the cylindrical wall of the chamber. The chamber is closed by a closure member which, when removed, allows the introduction of the diaphragm into the pressure chamber. The closure member is by means of threads screwed onto a sleeve-like projection upon the valve housing wider than the diaphragm. A central inward projection upon the closure member forms a stop delimiting the opening motion of the diaphragm. Said stop is displaceable by manually turning the closure member. A valve and a push button both situated centrally in the closure member control the pressure of the chamber.