It's been a while since I had it apart, but if I remember right, there are two "springs" in the cup. A wisbone-looking one that actuates the cams that lock it one way or the other, and a flat one at the bottom that provides tension to the whole thing. I had a similar cancellation issue on mine, and I got it working great by just cleaning the whole thing and regreasing, bending the arms on the wishbone a little to help it, and replacing the flat one with a little piece of spring steel I happened to have from some other long ago project. The only part I replaced was the wedge to hold the cup firmly, but that was cheap. Works like new now.
