Excluding issues with the brake servo, which I (fortunately) haven't had to touch, so consequently I have no knowledge of what's what in that department. In my case having the ABS warning light on and TRAC off were specifically associated with the ABS wheel sensor(s).   ABS issues are not always straightforward, but are usually associated with a few typical failures, and crap discs, crap/clogged relucters, and front brake sensors are the most likely. That said, the rear sensors, and a shed load