Kayfun Lite V2 can be leakproof if all the o-rings are intact. My test for a good seal is to remove the drip tip, cover the bottom airholes with a finger and thumb, and and draw a vacuum from the drip tip socket. If it won't hold the vacuum, I replace the deck o-rings. I never have juice leaks since I've included that test. O-rings wear as we remove and replace parts and the vacuum test is a necessary routine for leak free vaping. The usual culprit for a leak is the bottom o-ring that seals the coil deck to the tank. That's the one we remove and replace the most as we recoil and rewick. The center tank o-rings almost never need replacing because they usually don't get removed.
A little teflon tape around the fill screw can work some wonders too (thank you, @Bronze !).