There are 2 holes in the spring brake unit. The one closest to the plunger is your service brake and gets hooked to the plumbing you have now. The other hole is where the spring brake plumbing goes. You may want to use a quick release valve as a T. The quick release dumps the air when you release the pressure. Otherwise all the air will have to exhaust through the button on the dash. Kind of a quick overview: Air pressure to the service chamber applies the brakes. If there is no air pressure present, the springs apply the brakes for parking or emergency. While driving, you have constant pressure to the spring to keep it compressed so the service brakes can work normally. I wouldn't consider anything but plastic. Diesels vibrate and anything metal will chafe or crack in no time. Be sure to go to a heavy truck or trailer dealer. Someone like NAPA will be clueless as to what you are talking about. Besides, heavy truck dealers always have spring brakes on sale for around $40.
