Pneumatic booster

A pneumatic brake-booster comprising a casing inside of which is a piston having a back tubular part supporting a skirt and which, with the aid of an unrolling membrane, defines a front chamber connected permanently to a source of partial vacuum and a back chamber connected selectively to one of the front chamber and or to atmosphere via valve means actuated by a control rod capable of bearing, through the intermediary of a front face of a plunger, against a first face of a reaction disk attached securely through a second face to a thrust rod, the piston bearing against the reaction disk through a resilient annular front face deformable between a rest position and an active position, the annular front face of the piston being placed in the active position when the thrust rod exerts on the reaction disk a force greater than a pre-determined value, characterized in that the resilient annular front face of the piston comprises an annular resilient washer bearing through an outer peripheral edge against a step of the piston and against the reaction disk, and an intermediate resilient member disposed between the annular front face of the piston and the plunger.