Past parts might be worth looking at. They can bore and re-sleeve wheel cylinders, with a Stainless Steel Sleeve. Hence no further corrosion.
http://www.pastparts.co.uk/index.php?route=information/information&information_id=8I also had a problem with the rear brakes binding after about a seven mile drive. I replaced both wheel cylinders, but still had the same problem.
It turned out to be the brake master cylinder, once change the problem was solved.
