By the way, on some old Richdel valves, when you install a new solenoid, you want to remove the white plastic retaining ring before installing the replacement solenoid. That can make a difference in how well an older valve will open.
Just wanted to say thanks for this helpful advice. It's much appreciated. Our sprinkler system had lost pressure at a few different stations, and I was ready to rebuild with new diaphragms I'd ordered, but then caught your post and thought I'd give your advice a swing first. Sure enough, using solenoids without the retaining ring did the trick. Your advice saved me from a lot of sweat and headache, since the alternative, considering how and where the valves are located (1-1/2' underground) wasn't going to be an easy fix even for a minor rebuild. Cheers!