Honing brake master cylinder

OK, that's what I was afraid of. Probably my fault for letting it sit out for so long before getting to work on it -- I don't remember it being so pitted when I first took it off, but maybe it was just that I didn't know what to look for.

Well, anyway. I'm not totally committed to keeping this bike stock, so if anyone happens to have a recommendation for a new (front) brake master cylinder that'll work with the two-front-disc setup, I'm all ears!

Have you tried to clean it really well first? You will probably need a rebuild kit anyway, I would clean it thoroughly until it's shiny and seek if it has any leak with the new parts inside.

I've repaired ie: honed many a master cylinder that were a lot worse off than in your picture, I use a dowel rod chucked in a drill with a slot cut in the end of it and strips of wet/dry sandpaper. Start out with about a 220 grit and work up to 600, use rubbing alcohol as the lubricant and don't be shy, you want to flush away whatever is being removed. we are talking thousands of inches here not quarters of inches, the cups on the piston can well accommodate the small variation to a larger size,and with a rotational action the bore will not get out of round as someone has stated.
Even on a brand new master cylinder the piston does not just drop into the bore, the cups must be slightly compressed and that is to take up the space as the bore becomes larger just from usage.
The key is try to remove the minimal amount and still get the bore as smooth as possible.
90% of used ones will require this same procedure to actually be functional
Look at it this way, whats the worse that can happen it won't work but it doesn't work now :ohmy:

I had a crappy 14mm master on my LTD with dual discs and it should have been a 5/8" bore. Unable to find a 5/8" master in re-buildable condition, I drilled out the bore to 41/32" and sleeved it with a brass tube with a 5/8" id. I stuck it in with Lok-Tite 625 , drilled the passages into the reservoir, knocked off the burrs inside with 1000 grit paper on a stick and put in the new rebuild kit and re-installed it. It's been on the bike for 3 years and works great. Don't try this at home without some machining experience and tools. I did it with the help of a vertical mill and a lathe.

The problem with playing russain roulette with a questionable mastercylinder is that they tend to go bad at the worst possible moment in my experiance.
You can pick up a good mastercylinder for $30-$50.