Losing prime

[NDT888]

Hi there, I have a '99 Cummins CM570 Signature 565 that I'm having problems with. My fuel system was losing prime so I replaced the suction lines from the tank to the fuel housing on the engine. Also installed a Davco filter system on it at the same time. It was good for a while but after a while it started losing prime again but has plenty of fuel in the Davco filter. Like half full in the bowl. I replaced the injectors as it wasnt running good and after doing an injector test had a few of them that weren't holding and had bubbles coming back. Wondering if anyone has come across this issue and what did it take to fix it. Thanks!