To overhaul the Brake Master Cylinder, remove it from the vehicle and clamp the mounting flange in a bench vise (before service is initiated make sure that the rear of the primary Piston can be accessed). Clean the surrounding area of the reservoir cap and then you should remove the cap together with the diaphragm, check both of it for cuts, cracks, nicks, or deformation and changing the damaged parts. Remove the master cylinder reservoir and use a smooth and round-ended tool to apply pressure on the primary Piston and disingage the Piston retainer followed by the primary Piston assembly from the cylinder bore. Plug cylinder inlet ports and rear outlet port, apply low pressure, non-lubricated, filtered air to the front outlet port, and remove secondary Piston together with primary and secondary seals & return spring, discard primary Piston assembly, piston retainer & seals. Clean the master cylinder, secondary Piston and the return spring of the master cylinder with denatured alcohol checking for cracks, scores, pitting and corrosion to find any problems and replacing the master cylinder. Dry all parts with non-lubricated, filtered air and lubricate the master cylinder bore, secondary Piston, return spring, and all overhauled components with delco supreme 11 (R) (GM P/N 12377967) or a equivalent dot-3 brake fluid from a clean container. Put together the lubricated new primary seal and retainer, as well as the new secondary seal on the secondary Piston, followed by put in the lubricated return spring and secondary Piston assembly into the cylinder bore, followed by the lubricated new primary Piston assembly. Press the primary Piston down using a uniform, rounded tool to install the new Piston retainer, then reinstall the master cylinder reservoir and its cap and diaphragm before reinstalling the master cylinder back to the vehicle.