Denny is right on about the pits in the center of the bore. As far as how much you can take off I'd refer to a shop manual. You may want to try the online manuals if you do not have one for your particular truck. I think most of them is around .005 You stick the piston in the cylinder and then slide a guage down in there. It took very little to clean up my cylinders but, as Denny mentioned, I did not hone the ones out in the very center. I've bled my brakes ect and so far no problems.
For what its worth. If you do some shopping around you may find new cylinders for not much more than the wheel kits.....You can't just find the rubber cups...."maybe" at some old parts houses. Find out how late a model wheel cylinder is like yours and ask for that year model. You may get lucky.
