You can try different wick methods every time you pull it apart. When filling, turn the KFL horizontal and add until the juice comes half way up the window, any more than that is slight over filling and it causes dry hits/ bad wicking.
Sucks we can't take advantage of the full capacity but I guess when you operate on a vacuum you need some space in there.
Until you take it apart again, draw on it while covering the air hole and pop your finger on/off it a few times. Then watch for bubbles to float upward. This will help, but if totally filled up you will need to extract some juice.