ends at its apex in a seat 31 against which can be applied the movable element 22 which is fastened to a flexible membrane 32 at its periphery the flexible membrane 32 is clamped between a top body half 33 and a bottom body half 34 the top and bottom halves being connected by screws to constitute the valve body 19 in the bottom body half 34 the inside wall of the annular upstream chamber 29 forms below the seat 31 an inner passage 35 which leads to an inner space 36 fig4 at one end of which are situated the aperture 21 and the connector 21a in the top half 33 the flexible membrane 32 and the movable element 22 are free to move in the direction away from the seat 31 inside a downstream chamber 37 fig4 whose top wall has a passage 38 of small diameter which can be closed by a clack valve 39 the clack valve 39 is movable in a chamber 40 into which leads an inner passage 41 passing through the top body half 33 and the bottom body half 34 and thence into the inner space 36 the passage 38 and the valve 39 constitute an auxiliary cock which enables the downstream chamber 37 and the space 36 to be isolated or brought into communication on the clack valve 39 rests a rod 42 which passes through the wall of the valve 19 and which passes out of the latter under the lever 23 the lever 23 is pivoted on a pin 43 which is held in two lugs 44 fig3 spaced apart and forming.