Hydraulic control for automobile brakes

A hydraulic control, particularly for an automobile vehicle brake system having two separate independent circuits, in which control a master cylinder comprises a first chamber in which pressure is controlled by the driving-in of a master piston under the action of a brake pedal and against the action of a return spring, said first chamber forming part of a first hydraulic closed system comprising a first brake circuit, said first chamber being in constant communication with the first brake circuit so that throughout normal operation the force exerted through the brake pedal to the master piston is applied directly to the fluid in the first chamber and to the first brake circuit, the master cylinder also having a second pressure chamber forming part of a second closed system comprising a second brake circuit the said second chamber being adapted to be fed by a source of fluid pressure, the said master cylinder being provided with a one-piece plunger comprising a sliding valve member that controls said source of fluid pressure, two opposite end faces of said sliding valve member being subjected respectively to the pressures of the two closed systems, the master piston being disposed in the first chamber coaxially with the plunger in such a manner as to permit mechanical displacement of the plunger to actuate the second brake circuit in the event of hydraulic failure in the first closed system, the master piston cooperating permanently with the said return spring, and a second spring operatively disposed between said master piston and the plunger which is inoperative throughout normal operation of the hydraulic control when there is full fluid pressure in said first chamber, and, in the event of the failure of the first closed system, after the master piston has been driven in, with the second spring which only then acts between the master piston and the plunger in order to introduce modulating resistance simulating back-pressure, the plunger being mounted in a part accommodated in the body of the master cylinder, the master cylinder having a bore having a shoulder against which the said accommodated part is applied, an annular recess being formed on the first chamber side between the bore of the master cylinder and the plunger, and a seal in the annular recess with an outer face applied against the bore of the master cylinder and an inner face applied against the plunger.