diy overflow

[golf nut]

I suspect you are not priming at all, I don't care how you force water in the bubbles have the capacity to remain in the top of the U, drill and tap and install a john guest fitting in the first u tube, fill as normal then draw the remaining air out of the top of the U then submerse the JG tube below the surface of the water and you should be good to go, the principle on this system is sound.