glad you found the problem ! if replacing a couple of cylinders, replace them all. if the return springs are several [many ?] years old, you might consider those as well, and they aren't that much, cost wise. when you have the brakes apart, look closely at the backing plates for grooves, or worn depressions in the shoe contact pads. if any are found, a spot or two of weld, then ground down will fix that. small [tiny] depressions can be filed flat. upon reassembly, a dab of high temperature grease on these pads helps a lot.