A compressed air-operated servo-booster in which a housing (l) is divided into chambers (A, B) by a movable partition (ll) and valve mechanism (l5, l6, l7) controls the supply of compressed air to the chamber (A) for power assistance in a braking operation. The booster incorporates a reservoir (9), of which a wall (8B) provides fixing means (22) for a master cylinder to be assisted by the booster.