Pneumatic servo for boosted braking with silent operation

A pneumatic servo for assisting with braking, including a casing having an axis of symmetry, divided in leaktight fashion by a movable wall structure into a from chamber permanently connected to a source of partial vacuum, and a rear chamber connected selectively to the front chamber or to an outside atmosphere by a three-way valve actuated by a plunger sliding in a bore of the movable wall structure, said three three-way valve being connected to an axial control rod subjected to the action of a return spring located in an annular space between the control rod and a rear tubular part extending from the movable wall structure and projecting out of the casing, said three-way valve being located in this rear tubular part and including a shutter element interacting via a first annular surface with a first valve seat formed on the plunger and via a second annular surface with a second valve seat formed on the movable wall, said first and second annular surfaces being in different transverse planes, characterized in that said first and second annular surfaces are joined together by a surface having an overall frustoconical shape for guiding a downstream flow of air from said outside atmosphere as it moves past said first valve seat when said first valve seat moves away form said first annular surface and that said second annular surface and said frustoconical surface are formed on an annular element secured to an annular element on which said first annular surface is formed.