I know for a fact that these engines can run without an impeller and just off of the hose pressure. My guess is that this is what the shop was doing. Look for my thread on my pump not priming and read through the whole thing. I can't tell you for sure that your pump needs to be rebuilt (and that's the last thing that I would try) but I did outline and exhaust every other possibility and tried to document it as well as I could.
Former owner of a 1987 Supra Saltare. Current owner of a Malibu 23LSV.