About 50% of the time master cylinders are rebuildable with just a little clean up. No way of knowing till it's all apart and you take a look. You need to watch for pitting in the bore, and seals with rounded edges. It's difficult to describe on the seals, but if there's much in the way of large pits, the master will need sleeving. If the bore is ok, a rebuild kit should do it. There have been times all I've needed to do was clean everything up and put it back together to have it all fixed.