Our T&P valve has been leaking for months. We would just turn the cold water off to the water heater to get it to stop. We finally had our landlord install an expansion tank. It's installed, but the T&P valve still leaks continuously. The pressure coming into the house is 40 psi. I checked the pressure on the tank - it's also 40 psi. The expansion tank fills up as the water heats and expands, and when it's full, the T&P valve leaks until we use some hot water or turn off the cold water to the heater. Any thoughts?