Its funny that you said u get some "green stuff" with your extract because I have never gotten anything other than the honey oil color. PVC is not the best way to go IMO because of the fact that i may be getting a little bit of PVC with my oil(nothing noticeable, although my throat gets irritated more easily with this stuff?) As my thinking goes, Pressure is not what u are specifically after here. My guess isthat u pressurized to keep it as a liquid and help the effeciency of the extraction, but... The more pressure, the more crap that gets brought out with your oil. Butane is not "perfectly" selective. Yeah, u seem to be getting the best yields, but if u wanted stuff that u had to purify then just use acetone. Maybe, surround your tube with another container holding dry ice. All objects can be kept as a liquid by either pressure or temperature so this should allow all of the butane to leave your tube before it converts into a gas bringing all of the goodies that it can with it (maybe there is an ideal pressure that will bring the most oil with the least crap). Also, in theory, if u connect your tube firmly to the collection device, then as the liquid in your collection device begins to boil off, it can only go back into the tube where it gets cooled off(definitely have to use ice or dry ice for this to be worth it) once more, reconverted into a liquid, and emptied into your collection container. I may be missing something, but it should work, right?
P.S. Just trying to help not argue.