In a power brake booster for an automotive vehicle, with a housing (1, 2) subdivided into two compartments by a partition wall (41), the compartment close to the brake pedal contains a working chamber (25) and a vacuum chamber (33), while the compartment close to the master cylinder contains a working chamber (24) and a vacuum chamber (32), with a longitudinally slidable control housing (4) with valve device (8, 37, 38) being sealably held and guided in a central opening of the partition wall (41), the said control housing incorporating a longitudinal channel, for instance on annular gap (21) which, on the one hand, is via an opening or bore (26) in communication with the one working chamber (24), while, on the other hand, it terminates into a chamber, for example an annular chamber (20), which is connected to the valve chamber (42) behind the valve device (8, 37, 38) and which is connected to the other working chamber (25) via a cross bore (39). Through the valve device (8, 37, 38), the annular gap (21) is connected to another channel (34) which is provided in the control housing (4) and is in permanent communication with the suction port (11) so that both working chambers (24, 25) are connected both to the vacuum source and to the outside air through channels (34, 36 and 20, 19, 21, 26, 39, respectively) provided in the control housing (4).