Brake booster device, with improved operating conditions, for motor vehicle

A boosted braking device for a motor vehicle, comprising a master cylinder controlling the pressure in at least one braking circuit, a primary piston mounted to slide in the master cylinder to create a pressure variation therein, said primary piston being subjected to an actuating force made up of an input force exerted by a manual control member and a boost force exerted by a booster which is coupled to the manual control member and an emergency assist valve which comprises a reaction piston sliding in sealed manner at the rear of a first bore of the primary piston, said first bore having a front part in communication with the interior volume of the master cylinder, said reaction piston having an axial passage through which there passes a ratio control pushed by a plunger said, plunger being itself driven by the manual control member, the said ratio control at its front end comprising a head situated in a chamber lying between the reaction piston and a bushing arranged further forward in the first bore of the said primary piston, said bushing having a second bore in which a rapid piston of smaller cross section than the reaction piston can slide, said rapid piston also has a passage open at each end, a rear end of said rapid piston and a head of said ratio control forming a valve capable of closing the inlet to the passage in the rapid piston against the action of a spring which tends to open the valve, said boosted braking device being characterized by means for preventing a sharp retreat of the reaction piston when the manual control member is released after slow braking.