Road vehicle brake-pressure control device

A brake-pressure control device for a road vehicle equipped with an antilock system which during a braking operation regulates dynamically stable deceleration behavior of the vehicle, having a hydraulic multiple circuit braking system configured such that wheel braking is effected by operation of a brake pedal, a braking device comprising a master cylinder having output pressure chambers individually assigned to brakes and a pneumatic brake power assist unit having a vacuum chamber connected to an intake stub of a vehicle engine and a working chamber configured to be subjected, via a control element operable by the brake pedal, to a pressure higher than a pressure prevailing at the intake stub, a sensor to detect a position of one of the brake pedal and an element motively coupled thereto and to generate electrical output signals characteristic of at least one of instantaneous position of the brake pedal and variations therein, and/or a force transmitter which generates output signals characteristic of a force with which a vehicle driver operates the brake pedal when braking, and an electronic control unit operatively associated with the sensor and/or the force transmitter such that, by processing signals from one of the sensor and the force transmitter, output signals are generated by the electronic control unit as drive signals for the brake-pressure control device so that, during a driving operation, a higher brake pressure than the brake pressure otherwise to be expected in accordance with the instantaneous pedal force can be coupled into said brakes, furthermore, driving of the brake-pressure control device for the purpose of increased brake-pressure deployment being triggered when at least one of a speed at which the brake pedal is operated for the purpose of a brake pressure build-up and a rate of variation of the force at which the vehicle driver operates the brake pedal is higher than a prescribed threshold value wherein the vacuum brake power assist unit includes a solenoid valve arrangement driven by one of the electrical output signals of the electronic control unit and movable from a basic position, in which a control duct of the control element, via which pressure compensation can be performed between the vacuum chamber and the working chamber of the vacuum brake power assist unit, is communicatingly connected to the vacuum chamber, whereas the working chamber of the vacuum brake power assist unit is blocked off from the outside atmosphere, into an excited functional position in which the control duct of the brake power assist unit is subjected to the ambient pressure via a flow path of the solenoid valve arrangement that is released in the functional position, whereas the control duct is blocked off from the vacuum chamber, the basic position of the solenoid valve arrangement being assigned to a non-operated state of the braking system and to a target braking operation thereof and the valve arrangement passing into its excited position when, during operation of the brake pedal, at least one of the threshold value of the speed of operation and the threshold value of the rate of variation of the operating force is overshot, and a valve is operatively associated to respond to the pressure in the control duct, is subjected to an opening direction to a relatively higher pressure in the control duct than in the working chamber and is otherwise blocked, and via which the control duct can be connected directly to the working chamber of the vacuum brake power assist unit.