I have an older Model A shower valve. Was working fine. Got cold last night and now will not flow any water even in full on(hot ) position.No, it is not frozen. BUT,, when I open the lav faucett next to the tub the water flows full cold from the open shower valve.. Are these also sensitive to pressure? I have 4 other shower valves in the house and they are OK. Dirt? Limit stop? This happened overnight . Not gradually...

I have only worked on one flavor of Symmons shower valves, but it sounds like a stuck hot water regulator in the valve. They are supposed to maintain the outlet temperature even if the hot input temp varies a bit, and if the regulator gets stuck things don't work. Also I think these won't flow anything if the cold line doesn't have pressure. Check that too. If that is OK then rebuild the valve and things should be back to normal.

They are fiddly to work on, and you have to replace several pieces to rebuild them. Don't skimp and only replace the one seal. Use the whole kit even though it is expensive. I don't like them, but the other options for a temperature controlled valve are not cheap either, just easier to work on.