We have a Goulds V250 tank that is a diaphragm type. How do I check that type of tank? There is no visible air valve to add air.. The bottom sound full and the top empty, but I am no expert and was told that my bladder was bad on my diaphragm tank??? What focused my attention on the well system is that my spin down filter fills up with sand and has to be cleaned about once a week. In the past it has gone for months without needing attention. Thanks