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 of other problems are not impossible. Again in my case, I had a ABS failure light about 4 years ago, I replaced one tatty looking front sensor which made no difference, then replace a perfectly good looking sensor to have the problem magically resolve itself. This year a light came on I immediately order both front sensors (about £30 off of eBay) and the lights gone out. Okay maybe overkill, but you don't need yer GV in the garage for too long to have a bill far greater than what its worth.