the problems that the OP originally layed out are exactly problems I have experienced with filter drain back. the oil will drain out of the filter to the bottom of the threaded connection. that doesn't mean there "couldn't" be an air leak, but I seriously doubt that pressure would pick back up/normalize by mechanical priming if the system wasn't air tight. I've built milodon swinging pick-up engine combo's without issue, BUT the joint where the pick-up swings can leak air if not always covered in oil and that's why my preference is a static pick-up.
Just to mention, I do run a Moroso 3 quart accumulator. Even after turning it on and getting 24 psi out, the pump still won't prime.
You stated this started with a new filter - a Moroso - have you tried another filter yet?
As for the pickup sucking air - how is that possible when the pan’s oil level is above the swinging pickup ?
Also, the greatest contributor to a pump’s inability to prime is too much face clearance. Make sure no difference is a trans pump or engine oil pump. That and too much “resistance” to flow on the output side - hence your reduced micron level Moroso filter.
Been using Mildon systems since the late 60’s and never had an issue with any swinging pickup.
