Another dumb question . . . since it worked fine before you took it apart . . . is the hammer binding anywhere in the travel? Pivot hole in the hammer clean as well as the pivot screw? If it's getting weak near the "tail end", I'm just wondering if it's binding up somewhere?
If it worked fine before it was disassembled and the main spring was fine then . . I don't see how it would be weakened by sitting in a box. I would take it apart, check each part - slide the hammer over the hammer screw to see that it pivots freely . . . check the hammer slot in the frame . . . reassemble and see if it still doesn't work. It's probably something very simple that you just aren't seeing . . . and that happens to all of us! Sometimes I concentrate on trying to figure something out so much that I overlook "the obvious". Let us know how it goes as it may be of great help to the rest of us somewhere along the way!
Good luck and hopefully it will be an easy fix.
