Electric-servomotor vehicle brake system comprising: - a master cylinder equipped with a primary piston delimiting therein a primary chamber, connected by a brake fluid circuit to the wheel brake cylinders, and - an electric servomotor with an actuator piston acting on the primary piston, - a control rod connected to the brake pedal and moving a plunger piston to actuate the master cylinder with, when applicable, the assistance of the electric servomotor, - the movement of the control rod being detected by a sensor which informs the control circuit of the electric servomotor, - the master cylinder, the servomotor and the control rod being aligned on an axis (xx), wherein - the primary piston delimiting the primary chamber in the master cylinder is formed by a body * extended, on the outer side of the master cylinder, by a cylindrical skirt forming an outer chamber, housing a reaction spring of which one end is pressed against a retaining member secured to the skirt, * comprising an axial through-bore, connecting the end wall of the outer chamber and the primary chamber, - the actuator piston cooperating directly with the skirt to push the primary piston, - a hydraulic reaction piston, * housed in the bore of the body of the primary piston, * integrally bearing, on the outer side, an axial pusher, ** provided with a stop against which the other end of the reaction spring bears, characterized in that ** this stop bears against the end wall of the outer chamber to push the primary piston or be pushed by the latter. - a master cylinder equipped with a primary piston delimiting therein a primary chamber, connected by a brake fluid circuit to the wheel brake cylinders, and - an electric servomotor with an actuator piston acting on the primary piston, - a control rod connected to the brake pedal and moving a plunger piston to actuate the master cylinder with, when applicable, the assistance of the electric servomotor, - the movement of the control rod being detected by a sensor which informs the control circuit of the electric servomotor, - the master cylinder, the servomotor and the control rod being aligned on an axis (xx), - the primary piston delimiting the primary chamber in the master cylinder is formed by a body * extended, on the outer side of the master cylinder, by a cylindrical skirt forming an outer chamber, housing a reaction spring of which one end is pressed against a retaining member secured to the skirt, * comprising an axial through-bore, connecting the end wall of the outer chamber and the primary chamber, * extended, on the outer side of the master cylinder, by a cylindrical skirt forming an outer chamber, housing a reaction spring of which one end is pressed against a retaining member secured to the skirt, * comprising an axial through-bore, connecting the end wall of the outer chamber and the primary chamber, - the actuator piston cooperating directly with the skirt to push the primary piston, - a hydraulic reaction piston, * housed in the bore of the body of the primary piston, * integrally bearing, on the outer side, an axial pusher, ** provided with a stop against which the other end of the reaction spring bears, characterized in that ** this stop bears against the end wall of the outer chamber to push the primary piston or be pushed by the latter. * housed in the bore of the body of the primary piston, * integrally bearing, on the outer side, an axial pusher, ** provided with a stop against which the other end of the reaction spring bears, characterized in that ** this stop bears against the end wall of the outer chamber to push the primary piston or be pushed by the latter. ** provided with a stop against which the other end of the reaction spring bears, characterized in that ** this stop bears against the end wall of the outer chamber to push the primary piston or be pushed by the latter.