| 摘要 |
The invention concerns a power braking device for a motor vehicle, comprising a master cylinder (200) and a pneumatic servo boost (100) including a rigid outer cover (10) sealingly divided into at least two chambers (12, 14) by a mobile partition (16). The main hydraulic piston (30) of the master cylinder (200) itself comprises a hollow mobile cylinder (32) inside which slides a reaction piston (34) which comprises two-way valve means (94, 95) normally open, capable of interrupting the communication between the internal volume (V) of the master cylinder and the inside (35) of the mobile cylinder (32), and which comprises a first part (50) locked, in an inoperative position, against the mobile cylinder (32), and a second part (52) locked, in an inoperative position, against the first part (50), a third piston (82) capable of sliding sealingly inside the mobile cylinder (32), and being locked in an inoperative position, on the first part (50), and comprising a central opening (90) communicating the inside (35) of the mobile cylinder (32) with the internal volume of the master cylinder (200). The invention is characterised in that the third piston (82) is stepped and slides sealingly in a stepped bore (84, 85) of the mobile cylinder (32), the diameter of the front portion (84) of the stepped bore being less than the diameter of the rear portion (85) of this bore.
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