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As long as the Oring seats and the cup moves freely,, Its fine. Without the seal, the cup should move very freely.. Oil the seal before ya try to shove it in there.. Staining, corrosion on the outside dont matter, staining inside doesnt matter, but pitting where the seal runs isnt gonna be good once the seal gets to that spot,,It may seal, may not, might make it throough a few pads before that rough zone damages the seal enough to hurt it,,
Give it a go,, feel the lever move too easily once its bled of air,, look to see if fluid is coming out,, The picture makes me think it will go,,

  I'd try rebuilding them if they where mine.