Brake servo unit for motor vehicle, has rear thrust surface determining axial clearance having value that is maximal or minimal when master cylinder and piston are in positions corresponding to determined assistance ratios

摘要

The servo unit has a decompression piston (56) with a rear part (70) including a radial shoulder (86). The shoulder forms a rear thrust surface (88) that determines an axial clearance (A) with a front side (90) of opposite cups. The axial clearance has a value that is maximal or minimal when a master cylinder (14) and the piston are in positions corresponding to determined assistance ratios (R1, R2), respectively to avoid transmission of reaction force of master cylinder to a plunger (44) and a control rod (40), and the force is transmitted directly to a rear piston (28).