A valve piston (1) contained in a hollow cylindrical housing (3) is pressed against a sealing surface to block the flow of pressure medium from an inlet (11) to an outlet (12). The outer surface of the valve piston has a sliding seat supporting an annular brake piston (2). A contact element (13) protrudes from the valve piston outer surface. When the valve piston is in its position blocking the flow of pressure medium, the brake piston is spaced apart from the contact element so that in the case of excess load the contact element touches the brake piston. The brake piston dips into part (14) of the hollow cylindrical housing volume. A mechanical spring (5) dampens the axial movement of the valve piston near its end position. ADVANTAGE - The...