I've had that happen on a Kenmore and GE. I think the valves are built to pretty much have constant pressure against them, most folks leave the washer faucets open for years. I bet there is nothing wrong with the valve, and it will work perfectly when both hot and cold are hooked up and turned on, placing pressure on the diaphram.

On older thermostatic inlet valves since there is just one solenoid necessary when warm water is called for, this means that the warm solenoid and attached valve has access to both the hot and cold incoming water supply so this type of backup is very possible when the washer is not filing. To prevent this there is a check valve behind each inlet screen, it is a brass washer with holes in it with a rubber washer behind it to prevent back-flow. The problem is that these rubber back-flow washers are usually hard and have become ineffective long ago.

So if you are running the washer with just one hose, cap the other inlet, and when using the proper hook-up you must immediately turn off at least one of the water supply hoses to the washer when it is not in use or you may find that you are taking cool showers and flushing your toilets with hot water due to the normal pressure variations that occur in most home plumbing systems. Another solution is to get check-valves and install them on both the hot and cold water supplies to the washer.

John L.