Did you go as far as pulling the fuel valves in the delivery head... the part the hard lines screw to? There's the possibility that they're stuck. There's the main housing, a shim, spring, valve, valve holder and then the seal.
I know you said everything looks good, but when looking at mine i was worried about the fuel ports in the rear of the AL housing (to the cast iron part). I need to get some pipe cleaners or something to make sure everything is unobstructed.
At this point it may be easier and cheaper to buy a used, known good pump, since your mainshaft is trashed. They usually sell for anywhere between 100-200 + shipping.
-Todd