Okay, so on mine when I look down in the atty connector, I can see a screw with an o-ring directly below it. Is this not correct? From what you're saying, it sounds like I should see a washer, which I do not.
Also, none of the three IKV 306 atties I've been rotating will fit on this. They start to screw in but only get a few threads down and completely bind up.
I'm not familiar with the 306. Does that thread into the 510 cap?
Both of my caps are 510s. I'm not sure if the rest of them have a washer. The washer on the new caps are black and hard to differentiate from the o-ring by just looking down the hole. The washer should push down on the o-ring and compress it so no fluid gets through. I knew something was up with mine when I unscrewed a carto and the screw was leaning to the side. It was easy to wiggle the screw around with my screw driver as well.
I was able to free up the rough 510 threads by threading the offending adapter in and out several times. With an extra cap on hand I was able to throw caution to the wind. You might want to proceed a bit more carefully.