Patent ID: 6912850

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 ) slidably mounted in the master cylinder to create therein a variation in pressure, this primary piston being subjected to an actuating force made of an input force exerted by a manual operating member ( 4 ) and of a boost force exerted by a booster ( 6 ) coupled to the manual operating member ( 4 ), and an emergency assist valve (VA) which comprises: a reaction piston ( 19 ) subjected to the action of an elastic return means ( 22 ) returning it to a position of rest, the reaction piston ( 19 ) sliding in a sealed manner in a bore ( 20 ) of the primary piston, the front part ( 20 a ) of this bore communicating with the interior volume ( 5 ) of the master cylinder; a rapid piston ( 32 ) of a cross section smaller than that of the reaction piston ( 19 ) sliding in a sealed manner in another bore ( 33 ) of corresponding diameter; and a ratio control ( 27 ) actuated by a plunger distributor ( 10 ) itself driven by the manual operating member ( 4 ), the assembly being arranged so that, under emergency braking, a hydraulic reaction is exerted only on the cross section of the rapid piston ( 32 ), characterized in that the elastic return means ( 22 ) for returning the reaction piston ( 19 ) is arranged outside the primary piston ( 3 ), and in that a transmission element (T) is provided to transmit the return force of the elastic means ( 22 ) to the reaction piston ( 19 ).