The test for any RV pump is what you are going to do. Install a suction hose and put in a bucket of water. If the pump will not pull the water out, the pump is bad. Odds are the inner components have "dried" out and the pump needs replaced. You can save a few dollars rebuilding yourself, but it is best to just buy new. At a Service Center it would cost more in labor to rebuild so we replace. YEARS ago(more than 30) Rebuilding could be done for about the same price as replacing. IF your test shows the pump will pull water(you NEVER have to prime), then the suction tube from the pump to the tank has an Air leak, or you have a winterization valve turned OFF for the water pump suction side. Doug