1980 mack fuel issue

Hi guys I have a 1980 mack I'm trying to get running correctly. It starts and runs but when it idles for about 5 min it stumbles and stalls out. When this happens It cranks long and finally starts. I thought it was sucking air so I replaced all the fuel lines as I thought it was sucking air. The fuel filters are new and have been filled I've primed all the way to the injection pump and this still happens. I'm a mechanic but not a diesel mechanic. I understand how the engines work but when It comes to drivability issues I'm laxed. I'd like to exhaust all possibilities before I shoot the lock off the wallet for an injection pump

 

Yes primary filter is full secondary filter is a little low

 

It does it if the cap is on or off

 

It does it if the cap is on or off

 

This gives any other ideas the pick up pump is after the primary filter and before the secondary. And it is a mechanical pump