Remember back in the old days when we had to rebuild the mastercylinders and wheel cylinders on our cars? Brush Research came out with the ball flex hones and they are fabulous. They dont remove any appreciable material, but do lay a perfect crosshatch pattern to allow the piston seals to actually seal.
They are readily available. Sometimes just changing seals that ride against a really smooth bore wont seal all the way. And--if the master cylinder piston has alittle side load, the bore 'could' wear in a slightly tapered pattern. WE found that years ago especially with aluminum master cylinders became more prevalent in the later 70's in the automotive industry.
So if you have a troublesome sealing issue, try this. Beats buying a new cylinder.
Tom