pressure relief valve at water shutoff
So if the valve is relieving pressure, by leaking, you know it's doing its job. Now if it's leaking because of high pressure what could be the problem? Could it be a waterlogged expansion tank, or a lack of expansion tank altogether? High city pressure? Do you have a pressure reducing valve (PRV), and if so has it failed?
By the way you are talking about the T&P valve coming off your water heater, yes?
So many questions to be answered.