I dont have this problem with my melling pump, but i did with my Milodon oil pump and was told by Best Machine to either switch out the O ring on the end of the snout/shaft for a larger one that will fit or smear a small dab of rtv around the O ring. If the O ring leaks it can suck air. Dont use too much RTV, if it gets into the pump it will snap your oil pump shaft. Just a thought.

http://www.youtube.com/watch?v=aIxaesJ190U

http://youtu.be/lCZAaN7fFLg