I must be the unluckiest person when it comes to Kayfuns. I had this problem with my original KFL as well. So I finally opened my KF4 after Christmas, and I've been struggling with leaking seals. And I can't understand what's causing it. The top and bottom threads where the tank sections attach to the chimney, I get liquid slowly seeping through the threads and past the o ring (the threads where the bottom tank section screws on the chimney, and the top cap threads). I've tried everything. I replaced both O rings with several different o rings. First I replaced them with replacement o rings that came with my KFL, which feel like exactly the same o rings. I had the same issue. Now I've replaced the KFL o rings with o rings that came with my Russian 91%, which actually feel a little different. They're not as tight to screw on to, and feel a little more flexible. Well these o rings work way better, but if I run my finger across the seam, there will always be just a smidgen of juice at the seam. I've tried using the included M-tank instead of the 3 piece tank sections, and it leaks even WORSE with the M-tank.
I don't understand what could be the issue. I've inspected the threads and fit/finish of the tank sections, and while there are disappointingly some rough spots on these threads, there's nothing that looks like it should prevent a seal. I'm stumped. And this is exactly what happened with my KFL when I first got it. It would leak out of the threads of the included M-tank, so I replaced it with the M-tank that came with my R91 and that stopped the leaking.
With these R91 o rings, it works well enough that I can actually use and enjoy the tank, I just need to occasionally dry off the seam. And I've been thoroughly impressed with the performance of this tank. I really don't know what to do, I don't think I should accept this on a $180 atty. I guess I'll just keep trying every single spare o ring I have on hand and see if I eventually get lucky, otherwise I'm going to see how well Svoemesto does with their warranty.