If pressure relief valve is leaking and the problem is not the expansion tank, the valve may have sediment preventing it from closing.
To check this turn the boiler off and let it cool. Lift the manual pressure relief lever and discharge some water for a 3 seconds, letting the lever snap back into a closed position. The water should discharge strongly and be relatively clean. If the valve leaks slightly afterward this could be caused by sediment trapped in the seat causing small leakage. Open the valve again and discharge a second time.
If the pressure relief valve will not close at all, close the boiler water feed valve and remove and replace the relief valve.
If no water discharges from the pressure relief valve at all, the valve is plugged and needs to be replaced or there is no water in the boiler.
If water is leaking or dripping from a pipe. follow the leak back to its source and repair the connection where the leak is originating. This will require turning off the water supply to the house and draining the boiler system.