Always had the problem of fuel pissing out with a full or near full tank on right-handers, with an otherwise apparently perfectly sealed cap.

Wondering if this is a common problem and how it is even happening.

I admit I haven't really done my research, i believe it may have something to do with check-valves and charcoal cannisters? I'm off to dig up some literature on that now, but in the mean time thought i would just put some feelers out here, because it just doesnt seem to make sense that fuel can come out of a sealed cap!!??