I have an H.B. Smith & Co. furnace that is pretty old. Last year, our expansion valve was leaking water behind the furnace. Our oil dealer came in and drained the expansion tank and replaced the expansion valve. It has lasted almost a year, but now with the cold weather coming, the furnace is working more, and it is leaking again. The expansion tank was full again, and I drained it myself, but the valve is still leaking. Any advice?