I generally disconnect the return and flush the gearbox with the old pump before I install the new one. That being said, I would assume cranking rpm might not be enough to push fluid through the gearbox. Like I said though, I've never done it that way. Like the kid above posted, it takes a little while to get the air out of a power steering setup with the engine running. Might take forever to move fluid at cranking rpm. Good luck with it. FWIW I just rebuilt my first saginaw pump for my highboy. I ran across a serpentine setup from an 89 van 460. If you do suspect something's wrong with the pump, don't be afraid to take it apart yourself. They are very simple, and much like the pump in an auto slushbox.

-Jared