in the brake booster known in the prior art , the partition wall comprises a metal disc supported by a control hub made of a plastic material . the force exerted on the partition wall is transmitted through the hub to a force transmitting member actuating a master brake cylinder . according to the present invention the hub is composed of a metal sleeve transmitting the forces and a guiding neck of plastic material including all air guiding channels of the control valve disposed therein . this guiding neck can be manufactured much easier and can be made of a plastic material of just sufficient strength to meet the reduced force transmitting requirements .