If you haven't seen it, start by reading
http://www.irv2.com/forums/f107/vacu...ps-306571.html more tips in there than you can read in a sitting.
Are you sure tank is empty? Red light means you are getting power. There should be a tank sensor that trips when full might be hanging up. Does it ever run, then not build suction? Run forever? Something get flushed that shouldn't have been?