Try adding a tiny o-ring to the screw. The tiny screwdriver that came with the R91 has two o-rings that will fit. Also a small strip of teflon tape or dental floss just under the head of the screw would work.
I've tried an extra o-ring, still leaks. Teflon tape is my next recourse. My gripe is that I shouldn't have to, and why has no one else reported
juice leaking from their 510? Maybe they have. I'm realizing KEBO has a lot of problems and they're not putting the quality into their product. If you remember some of the deals on these russians when they were released, it's also clear to see that they're marking these things up quite a bit. If you're not selling them at MSRP, KEBO will refuse to sell them to you.
I'm certainly done with KEBO. I'd move on to the
kayfun but I have almost 200 bucks invested in these crappy russian 91's.