He's familiar with air & fluid flow & vacuums, retired Roush Racing Engineer. I subbed a tall, narrow glass vase for the bell to see what was happeninginside. It revealed a large bubble of air at the top that kept the water from rising fast or high enough to allow the siphon to fire (the water level inside lagged behind the outside by more than 2" regardless of the height of the bell). That seemed to guarantee an overfill. Shortening the stand pipe didn't help either. My thought was the water in the s-trap resisted the pressure from incoming (into the bell) water. So we started hatching a hoisting op so I could get at and cut off the s-trap. He had some tiny bits and suggested we try someting simple first. What a relief that it worked! So maybe it is the absence of pressure in the bell that makes it work.

True, the hole is vulnerable. I have assembled a 3/4" bulkhead and emergency spill pipe above the full line so any overfill will be redirected back into the FT. In times of my absence, I will revert back to constant height so I can sleep easier, for one I am around here most of the time anyway as I have retired twice.