The only thing I would add is that while you are cleaning and rebuilding do the same thing for the brake calipers.You do not need to replace your brake line. First, buy all the missing parts to get your master cylinder in working order. Then you need a rebuild kit for your master cylinder, it should include the rings and diaphragms. Remove your master cylinder from the bike and clean it out. Use a toothbrush to remove all the grime and rinse it out with soap and water. Let it dry (use compressed air), then rebuild it. Once everything is put back together, flush your brake lines with denatured alcohol a couple of times. This should clear out any remaining crud in your brake lines. Fill and bleed your brake lines with DOT4 brake fluid and you should be good to go.