That's always a good question. Here's an interesting observation, though. I replaced my vacuum module a few years ago when I applied vacuum to it and it would not hold. I thought my surging idle that happened when coasting on hot days (over 80) might be due to that. Even though it would not hold vacuum when applied with a hand pump the advance was still maintained as long as vacuum continued to be applied. I am wondering if there is a chance that it actually is not intended to hold vacuum once vacuum is absent at the ports. It also did not resolve my surging issue. None of the many parts I have replaced since that started have done anything for that.
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