Abstract: A slide valve comprising: a valve body having two axially aligned openings provided with means for connection in a duct; a drive rod; a valve member; and a backing member; wherein the valve member includes a hollow opening out to both faces of the valve member and fitted with a piston movable between a simple abutment at one end and a shoulder in the hollow at an opposite end, the piston bearing against said shoulder via sealing means, a return spring urging said piston towards said simple abutment, said piston including, at its simple abutment end, means for co-operating with a sloping ramp of said drive rod such that during closure of the valve, the piston is thrust against said shoulder of the hollow.