Servo boosters for vehicle braking systems

A servo booster assembly for a vehicle braking system comprising a housing, a pedal-operated input member, an output member, a movable wall for augmenting the force applied by said input member to said output member, said movable wall dividing the interior of said housing into two chambers, valve means controlling the differential pressure of said chambers, a hollow valve body of said valve means, means connecting said valve body to said movable wall, a guide means located in the rear of said housing, a bore wall of said guide means defining a bore of non-round cross-section, a part of said valve body extending through said guide means, and said valve body, when viewed in cross-section on a plane which is perpendicular to the axis of said bore, comprising a radially inner surface of circular shape and a radially outer surface of non-round shape in non-rotatably sliding contact with said bore wall in said housing whereby said valve body is prevented from turning relative to said housing about said axis, said radially inner and outer surfaces of said valve body defining between them a radially thickened portion of said valve body, and a fluid conduit provided in said radially thickened portion and communicating with one of said chambers.