Hard starting and Davco 382 leak down issue

I have tiny bubbles and hard starting on mine davco…I replaced the check valve and the whole clear cover and I still get it. Any ideas?

 

if all the gaskets are good and sealed i've heard mentioned a lot about cracked injector cups, allowing air into the system. but based on where the bubbles are, doesn't make sense to me. fuel goes from tank to small pump, through check valve with the little ball, through small filter, to davco, back to big filter, then to high pressure pump/rail. my bubbles coming from right above the davco drain valve inside the dome, next to the sensor looking thing with the two wires coming out of it. I think that's the inlet side of the davco, air from cracked injector cups wouldn't make it all the way backwards, upstream, to the davco, on the inlet side doesn't make sense. there has to be a way to pressure test the fuel system to find a vacuum leak between tank and common rail, it's somewhere pulling a vacuum and sucking a little air in. who is a trained diesel tech, they have step by step methods to isolate this stuff, there's a specific pressure/compression test for everything

 

maybe I’ll take everything apart one more time and clean the thread where the cover goes and I’ll put Teflon tape around thread where check valve screws on

 

Have a similar problem. How do you diagnose air in fuel lines? My Davco in not dropping fuel but my truck sputters/struggles for fuel for a few seconds after sitting over the weekend. If I use the primer pump everything is fine.
06 Volvo D12
Also is it normal to have fuel leaking at primer pump while using it?