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Update:
As you guys suggested, there was an air bubble trapped in the cooper tube/bulb. I hung the gauge with head higher than bulb for about a day, then cycled the bulb from hot to cold water three times and low and behold, a bubble appeared in the head. At that point, the gauge settled in at exactly 212 when the bulb was held in boiling water. The OEM gauge is a nice bright red too. My old one was a repro and the fluid in the head was nearly clear and hard to see. Thanks for the suggestions!