I just learned some more info on the pump, it was given to me for free but the previouse owner had some issues with it loosing prime and running alot. it ended up being some junk stuck in the check valve. he got fed up and replaced the pump in the proces. if the pump had run dry could this be the reasone it will not make enought pressure to trip the pressure switch off. the pump is in good shape when running is quiet and has no external leaks from the seals.how will i know if the impellers need replaced if i go into this pump.