[sammyg2]

If the problem in the master cylinder is ONLY the rubber seal you can rebuild it. BUT, based on the decription you gave on the fluid I would suspect it was contaminated and probably caused at least some corrosion in the M/C. If that is the case you should replace it. Rebuilding a master cylinder that has some pitting in the bore is not a reliable fix IMO. You could pull it apart and inspect it and then decide.