Rodger,

I assume you have a "normally-closed" valve, so it shouldn't leak when in the off position because it will stay closed. Maybe you are losing power to the valve or the valve is getting hot and closing on its on.

If you can put a voltmeter across the valve, you should be able to see if it is losing power during the draining process. If it does lose power, it could either be the valve or a problem in the circuit feeding the valve.

If you can put an amp meter in series with one side of the valve, you may be able to tell if it is getting hot and drawing more current than it should. If it does, you'll know the valve is bad.