Have you managed to get it actually going and then after half an hour or so it stops with a rattling noise from the impeller? If so, sounds like it's sucking air into the intake from somewhere. Check all the intake piping is clear and tight.
Have you never actually managed to get it going? Try pulling the outlet hose off the tank and siphon/run it into a bucket until it clears the air out.
The 400 series iirc had great plumbing for that, you could easily pop the inlet and outlet hoses off the tank.