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...