I am by no means an expert but have plumbed two filter systems in.
I hooked a system up for someone and couldnt get prime.
The gasket/o rings were missing from the valves. I have noticed that if their is something wrong with the unions you will suck air.

Maybe you should take a pic of the filter system and post it?

I would try and shut the main drain off and try to pull prime from the skimmer this si the easiest to catch prime

I also lost prime in my early days cause my water wasnt up enough and the skimmer was sucking air

And if using a hayward superpump i think their is a gasket on the cover that accesses the basket in the pump.

Hope this helps
Write back
Mike