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Fuel system does not retain a pressure........ it is a open circle from the tank right back to the tank...... If there is an air leak into the system then drain off will occur requiring a re prime.
Visit...... Ford diesel 6.9 7.3 IDI do some reading and a few tests..... you will find the air leak and solve the problem.
Do you still have the water separator on the firewall by the booster.......
If there is a leak (and it is likely) in the water seperator then it will be from the drain. You can plug the drain with a bolt of similar or bypass the seperator. I bet that is your problem.