BTW, I didn't see this posted above, but I might have missed it...

Have you ever checked to see if the brake system (MC, lines, calipers) is actually holding pressure when you aren't driving and the engine is off (taking the booster out of the equation)?

With the engine off, you just press the pedal over and over until there is pressure and then you press hard on the pedal and hold it there for 1/2 a minute or so. The pedal should stay firm and not drop. If it does drop, you might have a leak somewhere (could be real small) that is letting air in, but not letting enough fluid get out for you to notice the difference :dunno: