Brakes are a real touchy subject. And when I can't or couldn't find someone with new units I would rebuild them using the proper kits. If a Honda 600 master cylinder is leaking by one of it's seals there is most likely more than one reason. If the bore is smooth then something caused the seal to loose it's seal. You know what the polishing rag looks like after you polish your chrome or aluminum door handles on the sedan, that black sooty stuff that darkens the rag.
Well when you see your DOT3 brake fluid turning dark it's because the seals have been polishing the insides of your master cylinder. All that sooty stuff is causing the seals to abrade over time and they leak. There are most likely lots of shops out there that can do the job, but the ones I use is White Post Restorations (White Post Restorations [wpost@...] or www.whitepost.com ) are they the cheapest shop on the block? I don't know, they do have a warrantee that does not quit and they stand by their work. Brass Sleeve and new seals, rebuilt to factory specs.
If you like to take the risk that's ok, but think of the guy your going to bump into.
Just a thought, and Be safe.
Bill