There are 2 things that are intentionally left out, as they don't add value to the kit for the substantial increase in cost. These are the large square shaped ring where the solenoid meets the valve block on the diaphragm solenoid (blue wires). This mainly seals out dirt and water. If there is air leaking from around this ring, then check the o-rings within the solenoid. Also a diaphragm is not included, they fail on a very limited basis and are significanly more expensive than the oring kit. They are available separately in the valve block section.
Missing parts? 
