Who's fought Silverado/ Sierra brake problems? and won?

Well, and this is probably no help, BUT, going back to basics, anytime the pedal "goes down" or " is soft" is usually an indication of air in the system, or an internal leak, such as the master cylinder. I would think even with the electro snazzy vacuum pump boost thingie that if the brake boost was inadequate, you'd "still" get a very hard pedal, not "the other way", so "I" would be looking for a hydraulic issue.

Course I could be slap wrong as heck, too.