Hydraulic brake-power booster unit

A hydraulic brake-power booster unit for a vehicle having a dual-circuit brake system, that is also equipped with an antilock system that operates according to the drain principle, and having two hydraulic separate power sources individually assigned to the two brake circuits, one of said separate power sources being connected to the input of a first proportional control valve that is actuated via a path simulator and is developed as a sliding valve, the output pressure of said first proportional control vavle can act upon the pressure in one brake-circuit, the other one of said separate power sources being connected to the input of a second proportional control valve that is assigned to the other brake circuit and is also developed as a sliding valve, the output pressure of said second proportional control valve being utilized for the buildup of brake pressure in the second brake circuit, said second proportional control valve having an emergency piston that, in the case of a breakdown of the separate power sources, can be shifted only by means of pedal force to cause the buildup of a pressure in an output pressure space that is proportional to the pedal travel, wherein a piston arrangement is provided in a main bore of a brake-power booster unit housing between the emergency piston and an end front wall of the housing on the side of the pedal, said piston arrangement comprising an outer ring piston that is sealed off with respect to the main bore and a central auxiliary piston arranged to slide in a pressure-sealed manner in said outer ring piston, said piston arrangement, on the one side, together with the emergency piston delimiting a first working pressure space into which the output pressure of the second proportional control valve can be coupled to act on the whole cross-sectional surface of the main bore on the emergency piston, said piston arrangement on the other side, together with the end front wall of the booster housing on the side of the pedal, delimiting a second working pressure space into which the output pressure of the first proportional control valve can be coupled, to act upon on an overall surface of the ring piston and of the auxiliary piston corresponding to the cross-sectional surface of the main bore, wherein a slider of the second proportional control valve is developed as an essentially rod-shaped extension of the auxiliary piston arranged to slide in a central valve bore of the emergency piston, said section extending between a central piston flange sealed off with respect to the main bore and a piston flange of the emergency piston, delimiting the additional working space on one side, said flanges axially delimiting the input pressure space of the second proportional control valve connected to the power source, wherein a pressure rod part movably coupled with the brake pedal is arranged to slide in a pressure-sealed manner in a central bore of the end front wall of the booster housing, wherein for a drop in pressure in the second working pressure space said pressure rod part is supported directly at the auxiliary piston and, via said auxiliary piston at the emergency piston, wherein a path transducer is provided that generates an electrical output signal that is characteristic for the position of the emergency piston, and a pressure transducer is provided that generates an actual pressure value signal that is characteristic for the pressure in the output pressure space of the brake-power booster unit, and wherein a comparison arrangement is provided that generates a warning signal when the actual value of the pressure at the output of the brake-power booster unit is lower than a desired value of the output pressure to be expected according to the position of the emergency piston.