Abstract: A pneumatic brake booster (10) and a hydraulic master cylinder (60) are arranged in a series along a common axis (A) to form an assembly which includes a booster housing (12), a power input member (30) and a power output member (40). A mechanical power transmitter (32) is arranged between the power input member (30) and the power output member (40). A sleeve (50) is rigidly attached at a valve housing (22) to enclose a portion of the power output member (40). At the booster housing (12) a master cylinder housing (62) is attached which has a guide (68) facing the power transmitter (32). A primary piston (70) is guided in the master cylinder housing (62) and includes a support (72) for the power output member (40) as well as an extension (74) which extends towards the power transmitter (32) and is guided in the guide (68). The sleeve (50) extends to near this support (72) and is provided with a head portion (52) which bears against the master cylinder (60) via the guide (68) in a radial direction.