I think the bubble are coming from the rubber that holds the brass fuel tub into the tank. It comes out with 6 size festo tubeing into a 6 size festo Y the straight to the UAT were there is a 6 to 8 size festo brass adapter

The vent is all 6 size festo with a T, ( the second purple T in the vent is for the Smoke tank above) one drain port on the bottom.

I was thinking vent restrictions too and pulled all hose off the tank and the UST still sucked all the way in against the round paper filter

Thanks