My 1990 150 has a 4.9 and keeps losing prime. I have replaced the pump and filter and fuel pressure reg. There are no check valves in line and all the connectors seem good. Everytime it sits for a while I need to leave the key in accessory position till pump stops. Truck also seems to be losing power now. Could it be the the pressure side of the fuel line there are 4 push connectors on the line

 

I moved this to another forum so more members might see it. Nothing wrong with where you had it, just gets less forum traffic.

 

Is there any signs of leakage?

 

I cant find any leaks in the system. I tried dyeing the gas and looking with ultraviolet light and found nothing. The fuel pressure is 35 pounds and pretty much stays there

 

I had a similar problem with my '91. I had to turn the key toward and wait up to 20 minutes before fuel pump would stop and truck would start. Turned out it was a computer module, I wasn't able to figure it out and had a professional take care of it.