A master brake cylinder is described for a brake system of a motor vehicle as having a hydraulic cylinder that has a plurality of hydraulic connections, in which at least one hydraulic piston is mounted so as to be displaceable axially in a direction of actuation and in a relief direction, the hydraulic piston being displaceable against the force of a spring element in the direction of actuation, the hydraulic piston having at least one lateral wall opening that, in at least one sliding position, opens into an inner chamber of the hydraulic cylinder. It is provided that the lateral wall opening is assigned an elastically deformable sealing ring that on its inner side is fastened on the hydraulic piston and on its outer side is pre-loaded against the hydraulic cylinder by its inherent elasticity and is shaped such that when the hydraulic piston is displaced in the direction of actuation it detaches from the hydraulic cylinder, and when the hydraulic piston is displaced in the relief direction it bears on the hydraulic cylinder.