Master cylinder with improved emergency braking properties for a hydraulic vehicle brake system

A master cylinder ( 10 ) for a hydraulic vehicle brake system has a housing ( 34 ) and a bore ( 36 ) which is disposed therein and in which a hollow cylindrical primary piston ( 38 ), which acts on a pressure chamber ( 40 ), is guided in a sealing and axially displaceable manner. An axially displaceable, hollow cylindrical auxiliary piston ( 42 ) is accommodated in the primary piston ( 38 ), into which piston ( 42 ) an actuating piston ( 46 ) projects, which piston ( 46 ) is adapted to be actuated by means of an input member ( 24 ), is is guided in a sealing and displaceable manner in the auxiliary piston ( 42 ) and is resiliently preloaded away from the latter at least during braking. The actuating piston ( 46 ) has a first hydraulically active diameter (D 1 ) and the auxiliary piston ( 42 ) has a second hydraulically active diameter (D 2 ). The auxiliary piston ( 42 ) is rigidly coupled to the actuating piston ( 46 ) after covering a determined distance against its resilient preload. In order to achieve a brake assistant function, a shift piston ( 58 ) is disposed in the primary piston ( 38 ), which shift piston is resiliently preloaded in the direction of the pressure chamber ( 40 ) and is adapted to be displaced axially relative to the auxiliary piston ( 42 ). The resilient preload of the shift piston ( 58 ) corresponds to a predetermined pressure level in the pressure chamber ( 40 ), and the shift piston ( 58 ) is displaced relative to the auxiliary piston ( 42 ) against the spring preload once this pressure level has been exceeded, whereby a valve ( 68 ) closes the fluid connection between the second hydraulically active diameter (D 2 ) of the auxiliary piston ( 42 ) and the pressure chamber ( 40 ).