Today the water repairman returned to our house. We were finally successful in explaining the problem. When he turned on the water again, I can tell he was surprised at how fast the water pump turned on (less than 5 minutes). He went upstairs to the tank and saw the water lowering faster than it was filling. He came downstairs and pointed to where he thought the leak was.
Sunday morning they will be digging into our house to repair the leak. 🙂