Master cylinder for a motor vehicle hydraulic brake system

A master cylinder for a motor vehicle hydraulic brake system, having at least one piston, which is arranged in a housing so that it is displaceable along a central axis of the housing as a result of a brake actuation and to which a pressure chamber, which can be connected to wheel brakes, and a refill chamber is assigned,

• wherein the pressure chamber in the non-actuated state can be connected to a fluid reservoir via a compensating bore and on actuation of the brake can be isolated therefrom by means of a cup seal arranged on the piston in order to build up pressure, and the refill chamber is connected to the fluid reservoir via a refill bore,
• wherein the cup seal, on actuation of the brake, seals of the pressure chamber in the direction of the refill chamber and when the brake actuation is released allows additional fluid to be drawn from the refill chamber in the direction of the pressure chamber,
• wherein the cup seal is arranged in a radially open, encircling groove of the piston, which groove is formed by a first radially encircling projection and a second radially encircling projection,
• wherein when the brake actuation is released additional fluid is drawn from the refill chamber in the direction of the pressure chamber via the second projection,
• wherein the piston forms a captive unit with (i) a sleeve that is slidably connected to a screw element that is threadedly fastened to a threaded opening in the piston, and (ii) a return spring positioned between the piston and the sleeve to bias the sleeve away from the piston, and
• wherein the screw element is positioned through an opening in the sleeve such that the sleeve is configured to telescope relative to the screw element upon actuation of the brake, and a head of the screw element is configured to bear on the sleeve in the non-actuated state of the brake, and
• wherein the sleeve comprises a radially outward projection, upon which the return spring is mounted, and a cup-shaped member extending from the radially outward projection, wherein the cup-shaped member includes the opening in the sleeve that is slidably connected to the screw element, and wherein a central longitudinal axis of the cup-shaped member is radially offset from a central longitudinal axis of the screw element by a predetermined distance.