Patent ID: 6948313

Claim:
A boosted braking device, for a motor vehicle, comprising a master cylinder ( 2 ) controlling the pressure in at least one brake circuit, a primary piston ( 3 ) mounted to slide in a bore within the master cylinder to create a pressure variation therein, said primary piston ( 3 ) being subjected to an actuating force made up of an input force exerted by a manual-control member ( 4 ) and a boost force exerted by a booster ( 6 ) which is coupled to the manual-control member ( 4 ), and an emergency assist valve (VA) comprising a reaction piston ( 17 ) which slides in a sealed manner within a rear part of a bore ( 18 ) of the primary piston ( 3 ) while a front part ( 18 a ) of said bore ( 18 ) is communication with an interior volume ( 5 ) of the master cylinder, said the reaction piston ( 17 ) having an axial passage through which there passes a ratio control means ( 24 ) having a front end with a head ( 25 ) that is located in a chamber ( 26 ) lying between the reaction piston ( 17 ) and a bushing ( 27 ) that is located further forward in the bore ( 18 ) of the primary piston, said bushing ( 27 ) having an axial passage ( 28 ) in which a rapid piston ( 29 ) of cross section smaller than that of the reaction piston slides, said rapid piston ( 29 ) having an axial passage ( 30 ) open at each end, the rear end of the rapid piston and the head of the ratio control means ( 24 ) forming a valve ( 36 ) for shutting off an inlet to the passage ( 30 ) of the rapid piston ( 29 ), characterized in that said emergency assist valve (VA) includes means (E) for limiting a pressure difference between said chamber ( 26 ) lying between the reaction piston ( 17 ) and the bushing ( 27 ) and, front part ( 18 a ) of the bore ( 18 ) of the primary piston ( 3 ) which is situated forward of the bushing ( 27 ), to a value below a predetermined pressure differential that causes use the valve ( 36 ) formed by the head of the ratio control means ( 24 ) and the rear end of the rapid piston ( 29 ) to open.