If a check valve starts to go on the brake booster what would you expect for symptoms, in the brakes and the engine in general? What would a small leak vs. a faliure do?

I'm trying to understand the system and "How stuff works.com" has helped a lot! But knowing what the valve is supposed to do, that is make sure air ONLY gets sucked out of the booster, I am trying to understand what will happen to the system and car in general if air slowly leaks back into the booster over-time (usually overnight).

Might it cause engine idle problems under heavy break load? Why would the a "hiss" come from the booster area under heavy brake load or when the engine is shut off?

Thanks for the knowledge.

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Pete