of the annular sealing surface 48 is connected via a vacuum channel 44 with an annular chamber 61 of the changeover valve 32 the valve ring 55 is in abutment via the usual elastic reaction member 62 with the central push rod 17 which rod is located like the actuating push rod 53 on the central longitudinal axis 14 of the vacuum brake force booster 12 the piston wall 31 sealedly guided radially outward in the housing part 54 separates a pressure chamber 35 39 facing the actuating push rod 53 from a vacuum chamber 30 39 a readjusting spring 63 39 supported on a fixed internal partition 64 prestresses the piston wall 31 to its idle position as shown in fig1 behind the fixed internal partition 64 there is a further piston wall 65 which separates a further pressure chamber 35 from a further vacuum chamber 30 and is axially slidable within the housing part 16 consisting of die cast metal a readjusting spring 63 is supported on the rear wall of the housing part 16 and prestresses the piston wall 65 to its idle position as shown in fig1 the pressure connection of the pressure chamber 35 is ensured via a connecting channel 66 provided radially outward at the housing part 54 in the direction of the arrow f which connects the pressure chambers 35 and 35 39 with each other for connection of the two vacuum chambers 30 30 39 the piston wall 65 comprises a central sliding hollow pin 68 sealedly guided through the internal partition 64 which hollow pin 68 is fixedly connected with the thrust bearing 69 provide at the reaction member 62 for.