I don't know much about those pumps, but it could be that if the housing is cracked, leaking oil out, it may also be sucking air in. That will foam the oil, expanding it and make it run over. The air is compressible, so it will cut down on the pressure the pump generates. The pump looks a lot like the ones Chevy used in cars and trucks during the late 70s and early 80s. I've seen where they have been used to replace the original, but I don't know exactly what one is used. Maybe John can fill in the blanks here.