Brad, I'm unfamiliar with a valve on the side of the expansion tank ... it's probably the level sensor. It's important that your cooling system is leaktight, especially in your situation where you are taxing the cooling system with your steep hill. The cooling system is designed to be under pressure which increases the boiling point of the coolant. Since your cooling system leaks it cannot maintain pressure and the coolant is boiling off.

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Fred Hoelzle