The present invention relates to a reaction force transmission mechanism for a pneumatic brake booster including a largely incompressible reaction plate that is interposed between an input member and an output member and encased at the peripheral side and has lateral abutment surfaces for the input member and the output member, wherein the abutment surface close to the input member can be acted upon by a pressure member which is adapted to be fixed to a control housing and, consequently, can be excepted from the transmission of the reaction force to a vehicle driver. To improve its service life, the reaction plate includes a recess in the area of the abutment of a valve piston extension, in which recess a substantially incompressible, elastic insert is placed.