Hi. I am interested in this thread because I am also an SQ user. I bought two extra backs to use. One of them had badly decayed seals. I replaced them all with materials I bought from an English supplier I found on the web. I have also used Jon Goodmans products which are excellent. The thing I realised was that there are a large number of seals on these backs, any of which could cause a problem. It wasn't difficult, but more time-consuming than doing a 35mm camera. I would recomend that you get another back in good condition and try that. You can then spend time fixing the original. If that isn't an option, you could apply tape to all joints, including back to body, and darkslide slot, then take a test roll removing one piece of tape per frame to identify which joint/opening is the problem. Alex