Hello,
Yes, just one rubber o-ring near the top. Try going to the Tech Manual, Alfred has some good photos posted there. If the fragile lip at the top of the control valve has broken off, that is a problem. You can solder an entire new valve on the core if you can find one. I have seen emergency fixes for there valves with broken lips. some use a hose clamp with a slot cut into it. Clamp it on the valve so that the slot in the clamp performs as did the broken lip.
Lots of work getting in there. Best to fix it right.