a driver prior to activating the parking brake function has pressed down the brake pedal or because an automatic braking operation has been carried out then the hydraulic volume situated in the inner space 24 is isolated as represented in fig3 from the rest of the braking system 100 by closing the inlet valve 120 abs iso said hydraulic volume being so to speak shut in or locked in the pressure state prevailing because of the service braking operation is accordingly conserved in the inner space 24 at the start of activation of the parking brake function the electromechanical actuating device 34 is then triggered under the action of the electromechanical actuating device 24 the thrust piece 32 moves forward until the clearance is used up and the conical surface 46 is positioned against the corresponding conical surface 48 in the interior of the actuating piston 22 the result achieved is that the actuating piston 22 is supported axially via the thrust piece 32 and the thrust bearing 40 against the housing of the caliper 12 the thrust piece 32 then moves the actuating piston 22 further forward in this case the volume of the inner space 24 increases however as a result of the increase in volume of the inner space 24 the hydraulic pressure in the inner space 24 decreases this is because the hydraulic volume u2014 as previously described u2014 has been shut in thus unlike conventional solutions where the hydraulic pressure is kept at a constant level also upon displacement of the actuating piston 22 in the inner space 24 during activation of the parking brake function leading to a superimposing of hydraulic and electromechanical preloading the present invention provides that such a superimposing of hydraulic and electromechanical preloading is prevented the.