Trust me. I was going to say that.
No clue what is going on here but I know you both know.
sunnybob
13-Jul-2020 20:32
John, you have made a waste trap, if water gets sloshed into that trap the air will not vent. Cut the pipe at the bottom of the first downpipe. Then install a small air filter as used on motorcycle carbs to stop any creepy crawlies getting into the water.