check water pressure in house, the valves are made to releave pressure at 150 lbs. If pressure when leaking is normal its the valve, if high, likely to be a one-way-valve or regulator in line preventing backflow, and then you would need a pressure tank of sorts to have a place to expand to.... sence water expands when heated
http://www.factsfacts.com/MyHomeRepa...sureRelief.htm