I'm having a similar problem with my pump. I have ten years experience with this pump. So far I have found that it can be one of these problems,
1. It needs a new shaft seal and impeller.
2. The pressure switch is goofed up or needs replacement.
2. The tank pressure is goofed up.
4. Any combination of the above.
You can go to Sears.com, on the menu to the lower left click on parts, on the next screen enter the model number of your pump. That will bring up a screen where you can either view a parts list or a blown up diagram of the pump. Go to both and print them out to keep. (print the diagram landscape to get it all on one page). This will give you all the parts nembers and you can order a n owners manual.