I cannot get the return lines to blow out. I hooked my shop vac up to the line at the filter and turn the shop vac on but no air will come out of the lines to blow the water out. I did add a heat pump on this spring, and not sure if that makes a difference or not. I also disconnected the line at the back of the pool heat pump. Tried to blow it out there and it still didn’t work. All valves are still open. Does anybody have any suggestions?