Hydraulic booster

A full power hydraulic booster comprising a housing provided with a bore inside which is slidably mounted two pistons each associated with a respective pressure chamber connected to a respective brake circuit, an actuating piston mounted slidably inside the bore and displaceable by a brake pedal, displacement of the actuating piston causing displacement of the two pistons, and two valve means each associated with a respective brake circuit and controlling the flow of pressurized fluid from a respective pressurized fluid source to the associated brake circuit, characterized in that the two valve means are arranged parallel to the bore and are actuated simultaneously by the actuating piston, the pressurized fluid from the sources normally being communicated separately of the two pistonss and directly, via the two valve means, to the respective brake circuits, a failure of one fluid source resulting in the other fluid source providing pressurized fluid assisted displacement of one of the pistons which displaced fluid from the housing to the brake circuit associated with the one fluid source, failure of the other fluid source resulting in the other piston being displaced by means of the actuating piston so that the other piston displaces hydrostatic fluid pressure to the brake circuit associated with the other fluid source, the housing comprising a passage communicating the pressure chamber associated with the one piston with the associated brake circuit via a communication with one of the valve means, and a passage communicating the pressure chamber associated with the other piston with the associated brake circuit via a communication with the other of the valve means.