BREMSKRAFTVERSTÄRKER MIT VERBESSERTEM VENTIL

PCT No. PCT/FR96/01929 Sec. 371 Date Dec. 11, 1996 Sec. 102(e) Date Dec. 11, 1996 PCT Filed Dec. 4, 1996 PCT Pub. No. WO97/28032 PCT Pub. Date Aug. 7, 1997A pneumatic brake booster for providing braking assistance. The brake booster has an envelope (10) with an axis of symmetry (X-X') separated sealingly by at least one movable partition (12) to define at least a first chamber (14) connected permanently to a source of low pressure and at least a second chamber (16). The second chamber (16) being selectively connected to the first chamber (14) or a source of high pressure through a three way valve structure actuated by a control rod (30). The control rod (30) being capable of bearing, through the intermediary of a front face of a plunger (28) and a reaction disc (50) with a rear face of a thrust rod (48). The plunger (28) sliding in a bore (26) in the movable partition (12). The three way valve further including a valve member (36) disposed in a tubular rear part (22) of the movable partition (12) and co-operating through an annular front face (40) with a first valve seat (28a) formed on the plunger (28) and with a second valve seat (20a,20b) formed on the movable partition (12). The tubular part (22) having a space (35) located between the first (40) and second (20a,20b) valve seats and communicating with at least one radial passage (34) formed in the movable partition (12) which opens out into the second chamber (16). The tubular part (22) has at least one axial passage (37) which opens into the first chamber (14). The annular front face (40) of the valve member is urged in the direction of the first and second valve seats by a valve spring (42). The second valve seat (20,20b) being characterised by a surface formed on the movable partition (12) by bringing together at least two circular arcs (20a,20b) which are concentric, coplanar and of different radii.