It's been awhile, and I'm still having this problem. I bought a vacuum gauge and hooked it up to the the location that feeds the brake booster and the vacuum held.
It's starting to get worst because I notice it everytime I brake hard. When I come to a stop, the engine idles really roughly and usually settles down after 10 secs or so. Sometimes throws a misfire CEL because it's not getting enough fuel. I found that if I blip the throttle as I come to a stop things are okay and there's not a huge a/f correction.
Update: Another observation I made today. I was watching the short term fuel trims and noticed that everytime I got off the gas after cruising to coast, it would jump to +15-19% for a few seconds then settle back down. Looks like the braking just coincided with getting off the gas pedal. Anyone know why the fuel trim would jump up when coasting after cruising? Bad fuel pressure regulator? Clogged fuel filter?