When having disc brakes skimmed be care full as vented discs do crack when they get to hot, it is not unusual see disks split after they are skimmed, best option on vented discs is to replace with a quality product, i know as i work in the trade. have replaced all mine after only 10,000 miles it is not unusual for the callipers to stick this can cause the problem in the first place, mine are all copper slipped the (calipers that is) as the car only gets used at the weekend.