The fill whole spring might be stuck. Use the needle nose bottle that came with the kayfun and flush out the fill hole with warm water a few times and see if that helps.
If you still have an issue then try plugging the fill hole from the inside with a toothpick, problem with this method tho is that you will have to top fill the kayfun.
There is a little ball bearing inside, it sits in the bottom horizontally drilled section, the collar holds it in place. I used a pin to reposition it. Haven't used the fill hole since in the fear that it starts leaking again.
I tried using the fill hole when I got mine, couldn't get any juice in so just plugged the hole on the deck and been filling from the top since, not a single leak
Anyone know how to take out the ball bearing seams the spring not long enough channel about 10mm the spring only giving about 3mm tension
Or should I just pretty it closed