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.