Over the years I have rebuilt hundreds of master cylinders and many years ago, I wondered why my rebuilds (cast iron cylinders) failed after about three years of service – almost like clockwork!  At the time I was using a spring-loaded 3- (flat) stone honer, tiny but very similar to what a machine shop would use on an engine block during a proper rebuild.

Anyway, I dropped the darned thing on the floor and broke one stone, so as a replacement I purchased a carbide 'ball' honer and not one rebuild since then has failed.

The answer: flat honing stones cause the drilled ports in the bores to become VERY sharp and the cup/s take damage every time they pass over those ports.  By using ball hones, those passages are automatically chamfered.

I bought a full set of ball hones ... very costly (even back then) but well worth it.

Jim