I want to design a tubing system using flexible PVC (1/2" diameter) tubing that carries water in a loop. Obviously there is a pump too, but that's not the focus here. What I want is a way to get all the air out, so that the loop contains only water. There is the idea of a "fill port", where there is an extra tube stemming from the loop which you use to fill it with water, and allow air to escape. However, some manufactured products contain liquid loops that have no air at all, and don't have "fill port". How can I accomplish this?