Brake device for a hydraulic motor vehicle brake system, having: - a hydraulic booster stage, having a booster housing, and a booster piston which is mounted displaceably in the booster housing and which, here, separates a pressure chamber from a return chamber, wherein the pressure chamber and the return chamber are filled with a hydraulic pressure medium, - a master brake cylinder with a linearly displaceable pressure piston which generates brake pressure in a pressure chamber which is delimited by said pressure piston and which is filled with pressure medium, wherein the booster piston acts on the pressure piston with a boosting force which results from the pressure in the pressure chamber, - an electrically driven motor-pump unit which feeds pressure medium into the pressure chamber, - a pressure medium reservoir for the supply of pressure medium to the motor-pump unit, - a return connection which connects the return chamber to the pressure medium reservoir, - a piston rod which is actuatable by means of a brake pedal and which is guided in displaceable fashion by the booster piston and which has a hydraulic passage which, in a manner dependent on an axial relative position between the piston rod and booster piston, permits a hydraulic connection between the return chamber and the pressure chamber, - a first control edge which, in the open state, permits the connection between the hydraulic passage and the pressure chamber and, in the closed state, shuts off said connection, wherein the booster stage is provided in accordance with a circulation principle such that, in a braking situation, the pressure medium is supplied by the motor-pump unit directly, without an interposed high-pressure accumulator, to the booster stage in order to build up a boosting force that acts on the master brake cylinder, wherein a sensor device for the direct or indirect detection of position and/or movement of the piston rod is provided, characterized by an elastic reaction disk, which is arranged between the piston rod and the pressure piston and which, when the piston rod is actuated by means of the brake pedal, is compressible with generation of a reaction force, and in that between the booster piston and the piston rod, there is provided a locking means which, in a force-controlled manner, prevents an axial relative displacement of the booster piston relative to the piston rod and is designed to be releasable by actuation of the piston rod, the locking means being formed with an arresting piston which is moved transversely with respect to the piston rod and which is controlled by a pressure in pressure chamber, or being formed with a spring-loaded ball which engages into a radial recess on the piston rod. - a hydraulic booster stage, having a booster housing, and a booster piston which is mounted displaceably in the booster housing and which, here, separates a pressure chamber from a return chamber, wherein the pressure chamber and the return chamber are filled with a hydraulic pressure medium, - a master brake cylinder with a linearly displaceable pressure piston which generates brake pressure in a pressure chamber which is delimited by said pressure piston and which is filled with pressure medium, wherein the booster piston acts on the pressure piston with a boosting force which results from the pressure in the pressure chamber, - an electrically driven motor-pump unit which feeds pressure medium into the pressure chamber, - a pressure medium reservoir for the supply of pressure medium to the motor-pump unit, - a return connection which connects the return chamber to the pressure medium reservoir, - a piston rod which is actuatable by means of a brake pedal and which is guided in displaceable fashion by the booster piston and which has a hydraulic passage which, in a manner dependent on an axial relative position between the piston rod and booster piston, permits a hydraulic connection between the return chamber and the pressure chamber, - a first control edge which, in the open state, permits the connection between the hydraulic passage and the pressure chamber and, in the closed state, shuts off said connection,