Something's screwy with that Kayfun. I have an EHPro Kayfun 3.1, which is similar. I have one air hole and an allen screw air adjustment next to that. I've had slow leaking occur once and it was the o-ring inside the top cap. I replaced it with one from an aftermarket kit and it leaked worse, but that kit came from an Ebay source and the o-rings were different than the originals.
I scrounged around and found the box that it came in and there was a large o-ring just like the one in the top cap that was leaking vacuum. Replacing that fixed the problem.
You can also have serious leaking problems if the o-rings that seal the center sections have slipped and gotten cut slightly during maintenance. Sometimes, I will notice that my polycarbonate insert has stuck a loop outside when screwing the
tank back on. Be aware of that possibility. Even if you notice it, back off, and fix it immediately, there could be a cut in one of the tank o-rings.