As noted by Planetman, since new MCs are available, I would go with a new MC. As the master cylinder wears, it develops a ridge at the end of the pistons normal range of travel. To do a rebuild properly, this ridge needs to be removed with a hone or similar device. If the ridge is not removed, there is a high probability that the new piston will be damaged if / when it comes into contact with the ridge (pretty much guaranteed if you pump the brake pedal to bleed the system).

When I was very much younger (in a previous millennium) and into conserving money, I attempted an MC rebuild. It ended up failing about 2 months after I did the rebuild.

Repair kits are available, but I strongly suggest you buy a new brake master since they are available. However, the new brake masters no longer come with a brake fluid reservoir so you have to reuse your old one.

Yes they cost more, but when we, yes, we tried to rebuild one for a customer, we had problems and the #$%@ing piston assembly would stick and we blamed the rebuild kit.

We had to sell him a new brake master, which I prefer anyway as my liability is now limited to the installation in this case. Keep in mind I am a repair shop and I have to be mindful of my legal liabilities.

And FYI, so does anyone rebuilding a car part, so if you have an accident and it is determined that a rebuilt part was responsible, then there is a case for a suit.
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Eric
Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
Torrance, CA 90502