I bought a keg, followed the instructions here
for opening it, washed it out, and filled it partway with water. Now I'm having trouble resealing it. The spring will roll back in fine one loop around, but on the second loop, I can only get it in partially. This is causing the keg to leak, and has even let the spring pop out (fortunately, not while the keg was in use). I put the pipe/valve assembly back in, twisting the thing a fraction of a turn to line up the nubs with the notches, but it seems that it isn't going down as far as it should be.
Here's a rough mock-up of the notch on the keg, with the black bit being the nub on the pipe/valve assembly. Regardless of how much force I apply, I can't get the nub down an further. I think that if I could, that the spring would fit in properly.