Will an expansion tank and water pressure regulator work over periods of non-use?
When you get your expansion tank installed this leakage at the P&amp;T relief valve will stop. The pressure regulator will keep the pressure at your set point of 50 to 70 psi.

The pressure is currently building up in your system because as the water in the tank is heated (especially after a large drawdown) the VOLUME of the trapped water increases more than the volume of the tank as the water is heated in the tank. The volume does not increase indefinitely but stops when the water in the tank is heated to the set-point and the burner or heating element in the heater turns off.

You could prevent this increase right now by letting a cold tap drip until the heater turns off after large draw-down. Probably letting out as little as 3 or 4 cups out a cold tap would keep the pressure down. This would cause this same small volume of hot water to back up into the cold supply, but this is of no consequence.