Volvo 770 01,Cummins Isx,low fuel pressure goin' downhill & idle (air in fuel lines)

Hello, I have a problem with my 2001 isx cummins /770 volvo, the check engine goes on with "low fuel dellivery " or something similar and truck run rough ,it happens usualy when goin' downhill and sometime at idle.I was twice at Cummings for this issue and the first time they replaced the secondary fuel pump twice and the second time the Davco fuel separator,a lot of money and time for nothing since I still have the same problem.In the tests they did you can see clearly air in the transparent fuel line that they installed .
Thank you for your time and help.

 

Thank you Pablo , will look into that .

 

I just changed yesterday all the fuel lines and fitings from the tanks to the fuel pump,everything is brand new now and I still have the same problem.It does this also with the tanks full with diesel so I don't think that is the fuel pickup line from inside the tanks.If somebody have an ideea please Lmk.TY

 

Found the problem ,it was the fuel shut-off solenoid , the one that cut the diesel when the ignition key is turned off .