Most likely problem is the water section has leaked through the stuffing box, this 'gums' up the push rod above which (sounds like) is sticking the servo valve in the H/W demand position, this would indicate what you describe.
Solution: Replace the water section/stuffing box, clean the push rod above it, grease and replace. You will need to push the servo valve to one side to access the push rod.
Carefree, October 2010
