I once had a 440 that refused to prime after filter changes. Packed the oil pump with grease and it primed fine. Drove through two oil/filter changes and in a row with it acting like this until it suddenly lost all pressure a few minutes after starting. Pulled the oil pump and found small rust pits on the reliefe valve stem. Scotch brited them off and have run the engine for ten years and no problem since. Just a thought... Let us know what you find.