Hitting The Brakes

THE braking system on my son's 2007 Falcon XR6 sedan doesn't seem to react correctly. Sometimes it does seem to work, but at other times it seems to lock up or disengage. I went with him to see for myself and he stood on the brakes at 80km/h and they locked up and the car skidded sideways. The anti-skid system worked when checked by the dealer and there were no faults shown when checked by the computer. But when he got the car back the anti-skid system seemed to stop working again. He went back to the dealer and the mechanic told him he shouldn't need to jump on the brakes, so it shouldn't matter and that anti-skid system components are very expensive to replace. How should the anti-skid braking system react when braking heavily at highway speed?