I have 1992 Ford Explorer 4WD, the brakes seem fine around town but on a panic or quick stop the right rear wheel locks up and skids. The ABS does not show any trouble codes. I think it might be the Hydraulic unit aka proportioning valve. I would think something else would show up or the ABS light would go on. Any suggestions would be appreciated. Car is an automatic, 158K brake service includes recent master, shoes, pads and rotors, drums and wheel cylinders about 5K ago . thanks