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Anytime you've got dipping idle problems on any car, it's a bad idea to not check the throttle body for carbon deposits first. I'm not saying that it's 100% your problem, but it's such an easy, simple DIY thing to check that any car benefits from, it's just smart to start there.
Then, know that the vacuum pump is doing *something* with the air it's pumping, and that is sending it inside the crankcase, so when you brake, you're sending a shot of unmetered air into the crankcase, which gets sucked into the PCV and into the intake. If your MAF is dirty and already reading low g/sec, that incoming bypass of air might be "too much for comfort". While you've got your can of carb cleaner for the throttle (which you should only cleaned by hand with a cleaner-soaked rag, and not by spraying it directly), take the MAF off and give 'er a squirt.
If the vacuum pump wasn't working, you wouldn't have braking assist, it doesn't seem plausible that the pump would be working too much, so it's safe to assume the pump is fine.
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