Actuator piston receiving a hydraulic actuator linked to the control rod and acting via the reaction disk on the thrust rod, the actuator piston being subjected to the action of a return spring and of the drive of the servo brake, characterized in that A- the actuator piston is formed by an injection-molded piece consisting of a cylindrical body at the rear and a collar at the front, - the cylindrical body receiving the hydraulic actuator and the travel detector, - the opening of the collar opening into the cylindrical body being occupied by a shell covering the front face of the collar to serve as a support for the return spring and receive the reaction disk, the rear face of which is accessible to the hydraulic actuator, B- the rear end of the cylindrical body of the actuator piston includes an internal cruciform arrangement of primary lugs protruding toward the interior to form the guiding bearing for the hydraulic actuator, the front of the lugs forming an abutment, C- the hydraulic actuator includes a cylindrical surface that is bounded at the front by a cruciform arrangement of secondary lugs of a form complementing that of the body of the actuator piston and with an offset overall peripheral orientation, in order, on assembly, to introduce the hydraulic actuator by the rear into the cylindrical body and retain it by the abutment of the secondary lugs against the primary lugs after relative pivoting thereof, D- a locking and guiding sleeve, provided with a cruciform arrangement of ribs adapted to the cruciform arrangement of the intervals between the secondary lugs, is introduced into the actuator piston by the front and fits with the ribs (181, 181a) in the interval of the secondary lugs to block the rotation of the hydraulic actuator while allowing it to slide. A- the actuator piston is formed by an injection-molded piece consisting of a cylindrical body at the rear and a collar at the front, - the cylindrical body receiving the hydraulic actuator and the travel detector, - the opening of the collar opening into the cylindrical body being occupied by a shell covering the front face of the collar to serve as a support for the return spring and receive the reaction disk, the rear face of which is accessible to the hydraulic actuator, - the cylindrical body receiving the hydraulic actuator and the travel detector, - the opening of the collar opening into the cylindrical body being occupied by a shell covering the front face of the collar to serve as a support for the return spring and receive the reaction disk, the rear face of which is accessible to the hydraulic actuator, B- the rear end of the cylindrical body of the actuator piston includes an internal cruciform arrangement of primary lugs protruding toward the interior to form the guiding bearing for the hydraulic actuator, the front of the lugs forming an abutment, C- the hydraulic actuator includes a cylindrical surface that is bounded at the front by a cruciform arrangement of secondary lugs of a form complementing that of the body of the actuator piston and with an offset overall peripheral orientation, in order, on assembly, to introduce the hydraulic actuator by the rear into the cylindrical body and retain it by the abutment of the secondary lugs against the primary lugs after relative pivoting thereof, D- a locking and guiding sleeve, provided with a cruciform arrangement of ribs adapted to the cruciform arrangement of the intervals between the secondary lugs, is introduced into the actuator piston by the front and fits with the ribs (181, 181a) in the interval of the secondary lugs to block the rotation of the hydraulic actuator while allowing it to slide.