Abstract: A method of evaluating the operational characteristics of a brake booster by simulating a brake application of partially assembled brake booster components. An input force to control valve located in a hub of a shell to create an output force which is communicated through a reaction disc to an output member. An initial operational curve corresponding to an output force is compared with a reference curve to either accepted or reject the partially assembled brake booster components. If accepted, additional components are joined to the partially assembled booster to complete the assembly. If rejected, spacers based on the operational characteristics of the reaction disc and initial operational curve are located adajcent the reaction disc to modify the communication of a pressure differential developed force to the reaction disc for the now modified partially assembled brake booster components. An input force is again applied to the control valve to simulate a second brake application.