Wow JJJJ that is pretty crazy. Here is what I would try. I would try pumping water through backwards (in through the discharge). You could also try to cap the discharge and try filling the pipe with water from the vent I am assuming you have. The water should displace the air and sink the pipe. I still think there has to be, or was, air trapped in it. If you can fill the pipe with water where the water is rising up the exposed pipe the weight should push it down.