The invention relates to a master-cylinder having a two-part cylinder, one part being of narrower diameter than the other, and a two-part piston slidable in the cylinder. Two pressure chambers are defined by the cylinder and the piston. Advance of the piston reduces the volume of both. One, the high pressure chamber feeds an external pressure system, and is itself fed by the other chamber through a valve system during an initial phase of the piston stroke. In a later phase the feed from the second in the first chamber ceases and instead the second chamber feeds the fluid reservoir through a control valve. Pressure-transmitting means such as a rod is arranged between the high pressure chamber and the control valve to open the control valve when the pressure in the high pressure chamber exceeds a predetermined value. A safety valve is provided downstream the control valve whereby, upon abnormal advance of the piston, terminating communication between the second chamber and the reservoir and again feeding the first chamber from the second chamber. The safety valve is actuated by a force transmitting element upon engagement with an abutment fixed relative to the master-cylinder housing.