a tandem brake booster having first and second walls that move in opposite directions within a cavity to produce an output force . a lever arrangement attached to the second wall engages a fulcrum to transfer a force produced by movement of the second wall into the first wall . a control mechanism has a cylindrical body with an annular surface that engages the lever arrangement . the movement of the cylindrical body as compared to the output member is a function of the ratio of the distance from the fulcrum to end and from the fulcrum to the annular surface , and as such , maximum output travel can be achieved without requiring a corresponding input travel .