In overhaul the Brake Master Cylinder, first remove from the car and clamp the mounting flange into a bench vise, while keeping the rear of the primary
Piston accessible. Clean the reservoir cap area and remove the cap together with the diaphragm, check it for cuts, cracks, or nicks, and deformation and displace damaged components. Remove the master cylinder reservoir and depress the primary Piston by using a smooth round ended tool; withdraw the Piston retainer and the primary Piston assembly off the cylinder bore. Plug the cylinder inlet ports and the rear outlet port then use low pressure, non-lubricated, filtered air into the front outlet port to eliminate the secondary Piston along with the primary and the secondary seals and the return spring. Remove the main Piston assembly, Piston retainer, and secondary Piston seals. Clean the master cylinder interior and exterior, secondary Piston and return spring remove with denatured alcohol check for cracks, scoring, pitting, or corrosion and replace master cylinder if a problem is encountered. Allow all components to be dried with non-lubricated, filtered air and lubricate the master cylinder bore, secondary Piston, return spring and all overhaul components with delco supreme 11, gm p/n 12377967 (Canadian P/N 992667) or equivalent dot-3 brake fluid with a clean sealed container. Place the lubricated new primary seal and retainer (together with the new secondary seal ) onto the secondary Piston, then the lubricated return spring and secondary Piston assembly into the cylinder bore before setting in the lubricated new primary Piston assembly . Depress the primary Piston with a smooth round-ended tool, replace the new Piston retainer, reinstall the master cylinder reservoir and cap as well as the diaphragm before rejoining the sind (master cylinder) to car.