On my projects I always remove, disassemble, clean, sand and polish the castings, rebuild the master and carefully assemble everything. The castings can be polished to a mirror finish, if desired, but I stop at an 800-grit wet/dry sanding to preserve the factory sheen, then coat with a protective finish of Sharkhide. I also have the option of painting the brake and shift levers as the factory did, or grinding, sanding and polishing them to a chrome-like finish. In this case I want to preserve the original look, so these are painted with a silver/aluminum color, plus a satin clearcoat. This master cylinder is black from the factory, which required repainting, but later models wore a natural, unpolished aluminum finish.
Removing oxidation spots from the polished aluminum surfaces can be labor intensive, depending upon the degree of corrosion. In this case, all of this work took the better part of a day. In the left photo we see the right-side components for this 1986 VFR750 followed by both sides fully assembled, ready to install. TIP: Adjusting the brake light switch activation point is much easier if done before mounting on the bike.