An antilocking system for a road vehicle with static brake circuits operates on the principle of pressure modulation when brake pressure changes demanded by the control system remain small by changing the volume of a pressure modulator modulation chamber, which can be connected and disconnected to the wheel brake via an inlet valve. If larger brake pressure changes are necessary, the antilocking system operates on the pump-back principle, according to which brake fluid is pumped back via an outlet non-return valve from the modulation chamber into an output pressure space of the brake unit of the brake installation and before a further brake pressure reduction takes place in response to a renewed increase in volume of the modulation chamber. To permit switching from the pressure modulation to pump-back operation, the inlet valve is designed as a spool valve mechanically controlled by a hydraulically driven step piston, which forms the movable boundary of the modulation chamber. The spool valve takes up its shut off position when the piston reaches an end position associated with the maximum volume of the modulation chamber and only resumes its position connecting the wheel brakes with the modulation chamber after the step piston has executed a minimum stroke specified by the arrangement of stops on the piston and the spool and thus cause a reduction in the volume of the modulation chamber.