yeah you won't hear the leak.
spray some brake cleaner or something flammable that will evaporate on all your vacuum lines (make sure you don't get it on your headers or it'll burn!) if you idle surges, that's where your leak is.
also check the intake elbow - those are known to crack over time.
I can't remember what the brake booster/MC trick is..... sorry!
1992 BMW 325i (CHUMPCAR #296 - build in progress)
1992 3.0L M52 325i (ex-daily driver - track car only)
2002 330Ci (ex-daily driver - sold!)
Check out my e36 track car build