Fuel pump replacement..

I had the same problem. Replaced the fuel pump, then the filters again, the the pickup lines, then the primer pump, finally put a fass system on it, covered up the problem, but still lack little bit of power on big hill climb. Went back to Peterbuilt got another new mechanical pump and this fixed it. Then had to wait a month just to get them to return money on a new bad fuel pump. Wish I could get them to remburse for all the other parts that had to go through before going back to the orginal problem. They tried telling me new fuel pumps are not faulty. Now my Fass system (Only 6 months old) has bad pumpmotor, and going to take a month to get warrenty. Thankfully the truck will run without the fass system running and we did find and fix the orginal pump problem. Just cost me almost 2K to fix a $200 cat pump.
Your primer pump should also help overcome this issue, though it suck to have to primer everytime you shut the truck off or let it idle very long.

 

One of the causes for your problem is the hand primer pump. If it gets debris under the check valve it won't allow a vacuum to hold on the system and thereby allow the fuel to drain back to the tank. The mechanical pump internal seals can leak and cause the same problem.