To install tubing into a pp compression fitting, you must simply insert the tubing through the collet and O-ring, and into... moreTo install tubing into a pp compression fitting, you must simply insert the tubing through the collet and O-ring, and into the body of the fitting. The only trick is to make sure you have inserted the tubing all the way into the fitting. If you have not pushed your tubing past the O-ring on the inside of the fitting, you will experience a leak because the O-ring is not sealing properly. One way to be sure that your tubing has been inserted all the way is to mark the end of it at about the three-quarter inch line. When you press the two pieces together, the three-quarter inch line should disappear into the fitting. less