Fitted a new bottom inlet valve and was working fine. But then got a call saying the toilet was overflowing. Went back to take a look and found out then when its filling back up water is leaking from the large back nut that connects the arm to the main body. It shuts off ok so there is no grit on the seating but continues to drip out of the back of the large nut. It's fully tightened and it looks like it is sitting squarely on the seating, but continues to pass. Any ideas what could be causing this?.