As already mentioned, the nut on the stem may be the culprit here. Another thing to consider is how much muscle your putting into removing the pump from the stem. Your earlier posts of tube failure near the stem open the possibility that the tube is being damaged by too much pull when removing the pump. Earlier this year new pump cost me two tubes as well as cuts on my fingers. Both tubes failed at the base of the stem. That told me it wasn't the tubes. I switched to another pump, problem solved.