I would like to share an experience with you that hopefully will help others with the same problem.
I have a -3 which my father and I built back in the 90's, and we used proseal when assembling the tanks.
Fast forward to 2 months ago, when I noticed that when I went to fly, the fuel level in the tanks was very low, and in some cases, gone.
first thought was that the fuel thief gnome had been to visit, but quickly ruled that out. I use my inspection video camera with the long snake on it and checked behind the tanks, dry no discoloration. Also, no weeping rivets. After not flying for over a month, and banging an indentation in the wall with my head, a small light went on. I remembered that several months earlier I had replaced the fuel test ports with ones off of my -4 project, because I had noticed a little seepage from one of them.
Could it be that simple?
I removed the ports and replaced them with brass plugs. No leaks. I examined the ports and found that the little "O" rings were dry rotted, thus causing the leak. New ports ordered.
Lesson learned, even though they were never exposed to fuel, they still went bad. I should have just ordered a new set and been done with it, but because both tanks were affected, that thought never occurred to me. And it would have also helped if I would have entered into my log book the fact that I replaced them, a lot of the aggravation would have been avoided.
Feel free to chime in all you want, it was a dumb mistake on my part, but hopefully it will help someone else with a similar problem, or any other for that matter.
Time to go flying!