If it is corroded so bad it will not come apart, replacement is safest.

If you cannot get a replacement, honing may work for you for awhile. The means employed to get it apart are all reasonable if a good soaking won't turn the trick. Sometimes pushing the piston all the way in and giving the cylinder a good cleaning will allow it to come out.

One thing that often occurs is a little sand will get in past the dust boot (from flooding or just a bad boot). If so, the sand can get wedged between the cylinder and the piston. Lots of Brake fluid as a lubricant can be useful in teasing the crud out.

Try very hard no matter what method you use to get any crud out first or you will end up with an unrebuildable unit unless you have it sleeved.

Mike!