I had a leak on the heater valve the first in 25 years.
I bought a new valve. Tried to unscrew original in Head but seized solid.
Looked at past posts that suggest dismantle Valve and fit Diaphragm from new. So that’s what i did, too frightened to damage Cylinder Head.
100% sucess and great Tip.
Alan
Yep… I just restored mine (off the engine). I’m glad you could do it in situ.