By looking at the list of jets you own…I would assume you know how to setup and prime the system pretty well, but in case…here my taught..
‘’With Festo and Pisco connection, the tubing has to be cut perfectly 90* or there’s a chance of air leaking…..I also try to go into them square without a bow like at your valve in the picture.
When I prime with these filters….I prime extra long….and wiggle the filter a lot until no bubble come out of it, after that it’s primed and never have to do it again…..the first time you put fuel in any UAT or filter it takes a little time to take all the air out…the position of the filter has nothing to do..I have installed them in different attitude…..but horizontal is harder to prime correctly.
But I doubt the air bubble showed actually come from the filter it’s very large, unless you didn’t prime the motor and just started without prime ?? It look more like an air leak in a line…??