Vehicle brake servo booster with traction control mode

A brake servo booster for use in a vehicle hydraulic braking system comprising a servo piston which is connected to an output member and to which a supply of air is controlled by a valve mechanism which includes a reciprocable valve element connected to a force input member actuated by a driver-operated control device, the force input member including two parts of which a first one is connected, in use, to the driver-operated control device and the second one to the valve element, the arrangement being such that operating force applied to the control device is transmitted through both of said first and second parts of the input member by a direct mechanical connection therebetween to the valve element, whilst operation of the booster in the traction control mode is permitted by the second part of the force input member moving relative to the first part.