Help -Pump not pumping

[Sportsroof]

Hi All, I'm new here having just acquired a used Aquamist 1S system which I plan to fit onto a N/A 5.7 V8 with fairly high CR. Problem I have is that the pump initially did nothing when powered, and now I've got it making the right noises, it does not want to move any water. I tried priming it with a syringe and the water passes through the pump (powered) but there's no pressure and once the syringe is disconnected, flow ceases. I'm guessing that something inside is worn out/cream-crackered and I'll have to try to get my money back from the seller but if anyone can suggest a solution to resurrect this pump, I'll listen! Pump has PN 806-052 underneath if that helps at all.
Cheers, Martin