The pipe runs from the tank bottom to the Y valve which selects the deck fitting or the pump. The through hull is closed. What may be happening (assuming the issue is a blocked vent) is the loop vent is letting air into the tank when the Y is selected to the pump, venting the tank. When selected to the deck fitting I get flow until the tank has a vacuum equal to the pump out. What has me confused is the wife could hear water coming into the tank when I sprayed water into the vent.Just one question. You mentioned in your original post that you replumbed. Is there another route to the tank besides through the macerator pump? That is, is your Y valve before or after the macerator? If after, switching between the pumpout deck fitting and the overboard port, it would be trying to suck the water through the macerator, which it will not do. Just a thought.
Ken