Abstract: A pneumatic brake booster having a casing (10) with an axis of symmetry (X-X') and divided in leaktight fashion by a movable wall structure (12,20) into a front chamber (14) and a rear chamber (16). The front chamber (12,20) is permanently connected to a source of partial vacuum while the rear chamber (16) is connected selectively to the front chamber (14) or to the outside atmosphere by a three-way valve (36). The three-way valve is actuated by a plunger (28) which slides in a bore (26) of the movable wall (12, 20) in response ton an input applied to an axial control rod (30). The three-way valve (36) is located in a rear tubular part (22) of the movable wall structure (12, 20) which projects outside of the casing (10) and including a valve element (36). Valve element (36) interacts via a first annular surface (A) with a first valve seat (28a) formed on the plunger (28) and via a second annular surface (B) with a second valve seat (20a) formed on the movable wall (20).

Abstract: A pneumatic brake booster having first and second sources of air pressure for developing an output force. The booster has a rigid casing (3) divided by a leaktight moving partition (4) into a first chamber 3(a) and a second chamber 3(b). The moving partition being moved by a difference in the fluid pressures prevailing between the first chamber 3(a) and the second chamber 3(b) to drive a pneumatic piston (5) which carries a valve (7). The valve (7) being actuated by an operating rod (10). The inside (50) of the piston (5) is separated from the second source of air pressure by a purification filter (14). The operating rod (10) slides in a sleeve (16) and which passes through the purification filter (14) has a widened part (100). The widened part (100) together with one end (162) of the sleeve (16) forms an additional valve allowing an additional and direct air intake into the piston (5) in the event of an abrupt application designed to effect a substantial braking force.