your original problem with the master cyl is the normal, classic failure, where the o-rings begin to allow pressure bypass, and pressure seeps away.
example - you hit the brakes and initially they are soft, but you pump them a couple of times and they firm up. also - sitting at a red light with your foot on the brake and the pressure slowly bleeds away. a couple of pumps firms them back up. also - no brake fluid is actually lost.
so - do you think this is the same thing, or are you actually losing fluid. as suggested, check capliers and cylinders for signs of leaking - black, excessive dust, or maybe even wet. maybe spray them off good with brake cleaner and recheck in a few days. i'm guess its possible for them to fail, but i've never replaced a brake booster...
also - just give them a good bleeding. that never hurts, unless you brake a bleeder.
Last edited by dhvaughan; 02-10-2010 at 10:29 PM.