C15 Accert losing prime on suction side

I’ve got a 2006 C15 Acert that is getting air in the system on the suction side before the fuel pump. The pressure side of the fuel system will hold pressure fine. It started out the truck would lose prime overnight now it is only taking an hour for it to happen. The truck always fires up fine but starts to miss and die after 30 seconds or so. If I fill the primary filter before each start it run fine and doesn’t miss but if not it eventually will die. Everything I’ve read makes me think it’s sucking air somewhere possibly a bad line but with it losing prime so fast now I would think I would be able to see a wet spot somewhere that it’s letting air in. Any insight is greatly appreciated.

 

But where is there a check valve on the suction side of the fuel system? I can’t find anything from the tanks up to the transfer pump? Nothing where the fuel lines T together behind the cab and nothing on the primary filter housing.