It sounds like you do it all right. The only things I can think of is if the drum isn't fully seated (which is hard to believe), one of the brake springs was attached in the wrong spot and not keeping enough tension on the shoes, or one of the hold down springs has come apart. Check out the diagrams (Below). Please let us know if you need anything else to get the problem fixed.
Images (Click to enlarge)
Friday, September 10th, 2010 AT 11:30 PM