Control device of a brake booster, applying additional force when a pressure difference between a front and rear booster chamber exceeds a pre-determined threshold

A control device and a booster, the booster comprising a housing in which a drive piston separating a front chamber from a rear chamber can move in the event of an actuation of an input control member generating a pressure difference between the rear chamber and the front chamber, and an auxiliary drive member having a pressure chamber located in the rear chamber of the booster, the control device for exerting an additional boosting force automatically on said drive piston and located between the pressure chamber of the booster and a pressure source and further comprising a normally closed valve via which the pressure chamber communicates with the pressure source, and means for actuating said valve and which is sensitive to a pressure difference between the rear chamber and front chamber in order to open the valve automatically when said difference exceeds a predetermined threshold, the control device further comprising a hollow body in which is formed an inlet chamber for pressurized-fluid, a pressurized-fluid outlet chamber communicating with the pressure chamber, a first control chamber communicating with the front chamber and a second control chamber communicating with the rear chamber, said inlet and outlet chambers forming between them a first valve seat against which a first shutter is normally held to bear in a sealed manner by first elastic means so as to provide a seal, the first and second control chambers being separated in a sealed manner by means forming a piston which is axially aligned with the first valve seat and an extension of which forms a second shutter capable of coming to bear, so as to provide a seal, on a second valve seat fixed to the first shutter in such a way as, in a first stage, to shut off a communication between said pressurized-fluid outlet chamber and said front chamber when a pressure difference is established between said control chambers and, in a second stage, to move said first shutter away from the first valve seat, and second elastic means normally keeping said second shutter away from said second valve seat.