On Sun, 17 Jul 2011 11:07:29 -0400, Tony Sivori
wrote:

As luck would have it, the manual shut off valve leaks when closed. This
means that when the water supply is shut off, the tank slowly (about one
hour) fills itself to the full normal level.

This would be a great time to change the valve. Replace it with a 1/4
turn ball valve.

http://www.azpartsmaster.com/images/catalog/ishop/7070.jpg