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The cleaned bore. I wiped this out, then used very fine emory paper to smooth the bore. Ideally, you'd use a very fine brake cylinder hone. Or ideally, you'd rebuild the master cylinder before it was corroded. Cylinder wall corrosion was minimal, so I decided to keep the master cylinder. It's an expensive part. This needs to be spotless. Be especially careful to remove any grit from the hone/emory paper. I cleaned the cylinder with denatured alchol, and then with a clean rag and clean brake fluid.