Water Pump Runs and runs and runs ???

For years I thought it was normal to need to prime the pump after emptying and starting a fresh tank of water. I keep a couple of feet of clear tubing on the antifreeze inlet which is convenient for priming - just fill the hose with water, pump a few seconds then switch the pump over to the tank inlet. Oh, and the underfloor water tank had to be very full.

Then someone on this forum posted that the pump needed a new rubber part, the RV store guy knew exactly what I needed when I showed a phone photo of the model plate on the pump. Now the pump needs no priming.