Brake pressure control device for a road vehicle

A brake pressure control device for a road vehicle with a hydraulic multi-circuit brake system comprising a main cylinder for generating the brake pressures to be coupled into wheel brakes, a pedal-controlled pneumatic braking force amplifier operatively arranged to actuate the main cylinder, and operatively associated with the braking force amplifier for a switch-over device responding to actuation behavior of a road vehicle driver and configured to cause the braking force amplifier to be automatically switched over from a value of an amplification factor meeting a judicious braking requirement to a higher value of the amplification factor meeting a full braking requirement at least where a speed with which a driver actuates a brake pedal in an introductory phase of a braking action exceeds a threshold value, the braking force amplifier having a vacuum chamber connected to an induction connection of the road vehicle and, opposite the vacuum chamber, an actuation chamber movably bounded by an actuating piston and configured to be acted on by a pressure higher than a pressure prevailing in the vacuum chamber in proportion to a control force exerted on the brake pedal via a control part configured to act as a proportional valve and actuable by the brake pedal, the control part having a valve casing permanently connected to the actuating piston and axially displaceable therewith and a reaction piston forming a valve body and axially displaceable in the valve casing so as to be displaceable relative to the valve casing by actuation of the brake pedal in the direction of a force path marked by the control force and a resulting reaction force on actuation of the main cylinder, the reaction piston being supported by a reaction element, configured to be flexible at least in an axial direction, on a pressure rod for transmitting to the main cylinder an actuation force developed by action of pressure on the actuating piston, which reaction element exerts a reaction force directed in a direction opposite to the control force on the reaction piston and corresponding to a defined fraction of the actuation force acting on the main cylinder given by the reciprocal of the amplification factor of the braking force amplifier effective during the judicious braking operation, wherein in an axial force transmission train, leading from the brake pedal via the reaction piston of the control part, via the reaction element and via the actuating piston of the braking force amplifier to the main cylinder, for the control and actuation forces and for the reaction force resulting from the actuation of the main cylinder, which provides the driver with feedback on the functional condition of the brake system, a rigid force transmission element is provided in mechanical parallel connection to a flexible transmission element, with which force transmission element is associated a position engaged in the force transmission train as a basic position for the judicious braking operation, in which basic position the rigid force transmission element constitutes a distance piece between two force transmission train elements following sequentially in the force path direction, and a switch-over device is provided for the distance piece so as to respond when the threshold value of the pedal actuation speed is exceeded and then disengages the distance piece from a supporting position thereof and thereby frees a displacement path within which the reaction piston of the control part can be displaced against a reduced return force in the sense of a pressure buildup control of the control part,