fuel filter 03 duramax

fuel filter looses prime on 03 duramax when sits on hill. runs fine after i
manually reprime it. starts fine on level ground.

Look for leaks around the filter head. They can be rebuilt or replaced.
Check the vent valve.

Have you changed the filter and O rings lately? Those filter heads have been problematic. This fuel system has no transfer pump and relies on fuel pump vacuum to draw fuel up from tank. Any fitting up to the pump can be suspect. Housing drain valves,water in fuel sensor O rings,bleed valves all have o rings that can go bad. The schrader valve port ,(next to the alternator), is suspect too although this is a vacuum port. Keep a cap on this port. Pump the primer until it gets hard.Continue pumping and look for leaks. The filter head has check valves in it. If they go bad it can also cause this. There's aftermarket filter head replacement kits that eliminate this problem.