In an auxiliary power assisted master cylinder arrangement for a vehicle brake system to generate a controlled brake pressure in associated wheel brake cylinders, there is provided a brake valve with a preceding servo travel simulator to effect a control of the wheel brake cylinders of a vehicle. In one construction of the arrangement, there is interposed between two master cylinder units a pedal-operable device including mechanical cross links connecting with the master cylinder units as well as another cross link connecting with the brake valve. The brake valve is movable into engagement with the servo travel simulator, while the pedal-operable device, under normal braking conditions, is acted upon by the controlled pressure from an auxiliary power source in opposition to the pedal pressure and includes a lock providing for interlocking of all mechanical links in the event of a failure of auxiliary power. In another construction, each master cylinder unit is preceded by a second lock which is normally released with auxiliary power available and, in the event of a failure of the auxiliary power, couples the master cylinder units with the associated cross links at least in the direction of pedal operation. The present invention is advantageously employed as a brake slip control apparatus.