A process and apparatus for monitoring and controlling an anti-locking automotive brake system comprising a tandem master cylinder and an auxiliary pressure source, from which, in the event of anti-lock regulation, pressure fluid is supplied to the working chambers of the master cylinder. Auxiliary pressure regulation is performed with the aid of central valves and the auxiliary pressure source is turned on and off in response to the advance of the primary piston, a maximum distance being preset between the primary and the secondary piston. Once this space is exceeded, the central valve disposed in the primary piston, opens resulting in another advance of the primary piston and a switch-on of the auxiliary pressure source. If the primary piston after the auxiliary pressure source having been turned on, is not restored within the predetermined time interval, an error signal will be generated. If the way switch disposed on the primary piston fails to respond after a predetermined number of regulating cycles or pressure decreasing pulses, an error signal will also be generated and the anti-locking regulation will be turned off or the auxiliary pressure source switched on.