[wingnut60]

Have you drained the tank via low-point drains? This may be necessary to get the air bubble back into the tank. Having an air cushion on the top of the water will prevent the pressure from rising to the point the valve leaks. A completely full tank of water will try to expand as temps rise--having the air in the tank will compress some and keep the pressure on the valve lower. Water can't compress and will push against the valve as the temp rises. Can't hurt to try this if hasn't been done already. Otherwise, you are probably going to have to break the valve loose. There are several more effective rust dissolvers than WD40--also heating carefully with a small torch on the valve body may break a rust lock loose.
Joe