Pitch it and buy a new one. I have been through all those dangerous
exercises with compressed air and blowtorches and massive wrenches, to be
followed by hours of cleaning, inspecting, and rebuilding. Then when all is
done you put it in the car and it fails in a few weeks or less. In almost
all cases the only hydraulic cylinder worthy of being rebuilt is one that
is nearly perfect already, with no internal pitting. When you have one that
is really difficult to take apart it will almost certainly be severaly
rusted and pitted inside.
Rebuilding one of the old ones by simply honing out material to a point
where there are no rust pits is dangerous. Resleeving is necessary. In the
case of very expensive or irreplaceable cylinders then White Post
Restorations in Virginia or one of several other qualified shops will hone
and sleeve your old cylinder and by almost all accounts that I've heard
they do excellent work. In the case of most cylinders on more common cars
the replacement is the less expensive way to go.