My shallow well water pump is made in China which I bought about 12 years ago. It used to work fine and the pressure was good until a few years ago, the pressure won't rise above 10 psi and it won't shut off so I connected a switch so I can just shut it off when not in use. I have replaced the electric motor bearings with new ones (two of them) and replaced the mechanical seal and capacitor (8 microfarad). But still the pressure is below 10 psi and the motor won't shut off. The pressure switch is also brand new and is properly adjusted. The only thing left to replace is the brass impeller. There are no clogs or breaks in the impeller but I'm not sure if it is still good. How do I know if the impeller needs to be replaced with a new one? Does the impeller decrease in size after prolonged use? Thanks in advance to anyone who can give me some advice and share his or her experience with regards to impellers. My impeller looks exactly like this: impeller pump - Google Search (http://ht.ly/jZb23)