Scarecrow - When you had the pump apart, was the back plate or cover plate scored / worn much?
Mine worked OK, but I used to have to close the seacock, while sailing, or it would lose it's prime as well. After I reversed the scored cover plate, it works better, and I never have to close the seacock now.
I'd bet that if yours is worn somewhat, you could improve it's efficiency, by replacing the worn parts
(or flipping the cover over). It may be enough of a gain, to make a difference!