I have a 02, 2500 HD, Duramax engine that is losing prime. I have replace both rubber fuel lines coming from the fuel filter. I have replace the fuel filter twice, once with an after-market and now back to the OEM filter. I have replace the water sensor at the bottom of the fuel filter. I have been told that it might be my fuel pump which has a one-way flow valve that has become stuck allowing fuel to flow back to the tank. I'm out of ideas. If I let my truck set around 6 hrs or overnight, when I crank it, it runs around 20 seconds then shuts off and I have to re-prime it. The problems has gotten worst over time, I used to just park my truck with the engine facing downward on a slight grade and I would not lose prime, but now I'm losing prime everyday. Does someone have any advise?