More info about ABS.....A standard Anti-lock Braking System uses four wheel speed sensors, a hydraulic pump, four hydraulic valves (assuming it is a 4 channel setup), and a controller. The controller in the ABS system monitors the speed of each wheel using the wheel speed sensors, which are usually hall effect sensors. If the controller sees that one wheel is decelerating at a rate that couldn't possibly correspond to the vehicle's rate of deceleration, it actuates the hydraulic valve in that wheel's brake line to reduce the brake pressure applied to that wheel. This allows the wheel to turn faster. Once the wheel is back up to speed, it uses the pump to introduce the pressure back into that brake line, applying that brake again. When the controller sees rapid deceleration again, it activates the valve and the cycle repeats about 15 times per second. This happens the same on Disk/Drum and/or Disk/Disk.