POWER-ASSISTED BRAKING DEVICE WITH A VARIABLE ASSISTANCE RATIO

摘要

A motor vehicle power-assisted braking device including a master cylinder (200) and a pneumatic servo (100) operated by an input force applied to a control rod (26). The servo comprises a rigid shell (10) sealingly divided into two chambers (12, 14) by a movable partition (16) capable of being biased by a pressure differential between the two chambers (12, 14) resulting from the opening of a three-way valve (24). The main hydraulic piston (30) of the master cylinder (200) in turn comprises a movable hollow cylinder (32) having a secondary hydraulic piston (34) sealingly slidable therein, and communicating with the inside (V) of the master cylinder. The secondary hydraulic piston (34) comprises a two-way valve means (56) which is normally open but can seal off the inside (V) of the master cylinder from the inside (35) of the movable cylinder (32) when the change in the input force exceeds a predetermined value. Said secondary hydraulic piston (34) is composite and comprises a first portion (50) which, when it is in its home position, is urged into abutment with the movable cylinder (32) by first resilient means (46), a second portion (52) slidable relative to the first portion (50), and second resilient means (54, 92) that exert a second, rearwardly directed resilient force on said second portion (52) in order to urge same into abutment with the first portion (50), when said second portion is in its home position.