Servo brake system comprising a decompression piston integrated with the primary piston of the master cylinder

A boosted brake (10) for a motor vehicle having a pneumatic brake booster (12) for actuating a master cylinder (14), said booster (12) comprises a rigid casing (16) inside which moves a transverse partition (18) sealably delimiting a front chamber (20) subjected to a first pressure (P1) and a rear chamber (22) subjected to a second pressure (P2) varying between the first pressure (P1) and a pressure (Pa) greater than the first pressure (P1), a moving piston (24) fixed to the moving partition (18) and comprising a front face (26) which acts on a primary piston (54) of the master cylinder (14) by way of a reaction disk (28) housed in a cage (30) interposed between the moving piston (24) and the primary piston (54), a control rod (32) which moves in the piston (24) selectively as a function of a first axial input force exerted forward against a return force exerted on the rod (32) by a return spring (34), a plunger (36) which is arranged in front of the control rod (32) in the piston (24) and having a rear end (38) at least one annular rear seat of a three-way valve (40) which moves progressively between a position in which, with the control rod (32) at rest, the front chamber (20) and the rear chamber (22) are in communication, and a position in which, with the control rod (32) actuated, the second pressure (P2) prevailing in the rear chamber (22) increases, the valve (40) places the rear chamber (22) in communication with the pressure (Pa) which is greater than the first pressure (P1), and a feeler (42), forming a front end of the plunger (36) and passing through a bore (44) leading from the piston (24), which, when the control rod (32) is in the rest position, is arranged at a defined jump distance (d) from the reaction disk (28) and when the control rod (32) is actuated with an input force whose intensity is greater than a first defined intensity comes into contact with the reaction disk (28) to transmit to the plunger (36) and to the control rod (32) a reaction force of the master cylinder (14), the ratio of the area of the reaction disk (28) in contact with the cage (30) to the area of the feeler (42) in contact with the reaction disk (28) defining a first defined boost ratio, said cage (30) comprises at least one moving decompression wall (46) which, when the control rod (32) is actuated with a second axial input force whose intensity is greater than a second defined intensity greater than said first axial input force, said control rod moves to create in the cage (30) an additional volume in which a front part (48) of the reaction disk (28) expands to reduce the reaction force transmitted to the feeler (42) by way of the rear face (50) of the reaction disk (28), the ratio of the area of the reaction disk (28) in contact with the cage to the area of the feeler (42) in contact with the reaction disk (28) defining a second boost ratio, which is greater than said first boost ratio, characterized in that said primary piston (54) of the master cylinder is arranged in contact with the reaction disk (28) and said primary piston (54) comprises:

• a rear section (56) having a rear face (58) arranged in contact with the reaction disk (28) and in which an opening (60) leading into the rear face (58) that accommodates a matching decompression piston (62), of having a defined transverse size (E), which comprises at least two radial branches (64) and a rear face of which forms the moving decompression wall (46),
• a tubular front section (66) of the outside diameter of the primary piston (54) of the master cylinder (14), of which a bore (68), which leads out at the front end of the opening (60) and has a diameter (D) smaller than the defined transverse size (E), that receives a helical return spring (70) accommodating a return rod (72) of the decompression piston (62), in order to provide a decompression piston (62) of maximum transverse size (E) and said helical spring (70) of a minimum diameter (D) tailored to that of the primary piston (54) of the master cylinder (14).