Danny - your post is exceptional. This is exactly the way I approach repairs. My relief valve is spewing like crazy and (after shutting off the ball valve) while I was waiting for a call-back from some sprinkler guy I walked through your instructions. I opened mine up and found that the head of the diaphragm stem (flange that holds the o-ring that seals it at the relief port) was quite bent. As soon as I get a replacement, we'll see if that fixes it.
Anyway, thanks for taking the time to post this info.
Tom