Lockable master cylinder

A master cylinder, comprising a body having a bore in which slides in leaktight manner at least one piston controllable by a control rod and movable between a rest position and at least one working position and defining at least one pressure chamber connected to an application fluid system, said piston having a peripheral groove, the master cylinder including external means for selectively actuating a finger which enters said groove to lock said piston in one of the positions and maintain the one position of the piston when the control rod is in a rest position, the peripheral groove connected to a reservoir of fluid under low pressure and the finger locking said piston in a working position.