A pressure reducing control unit for brake system connected in the pressure connection between a master cylinder and a slave cylinder is known. The known arrangement is, however, of great overall length and is not safe against failure as well as being expensive with regard to production. According to the present invention the differential piston is constructed as a hollow cylinder having one end thereof slidably sealed in a housing bore and its other end slidable sealed on a plug fastened to the housing and projecting into the bore of the hollow cylinder. The pressure medium inlet is arranged in the plug. The differential piston on the end remote from the plug has a radial shoulder directed inwardly acting as a valve seat and separating an inlet chamber from an outlet chamber. The valve closing member is disposed at the end of the plug adjacent the shoulder.
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