When i have handle set to 'filter', the suction starts out fine, but after few minutes it gets weak. Figured I had air leak. I've checked all connections, made some tightenings, changed hoses, cleaned DE filter, but still suction and return is weak. However, when I set filter to 'backwash' or 'reciruclating' modes, the pressure is perfect. Could there be a problem with the filter handle setting? Could the interior get worn out?