RABS Open System
The Rear ABS is what is called an "open" hydraulic system. When an ABS stop is initiated and wheel lock is detected, the isolation or inlet valve closes, preventing any more fluid from going to the rear wheels.
Then, if necessary to prevent lockup, an outlet or dump valve opens to bleed fluid out of the rear brake lines. That fluid goes to an accumulator and is held there for the remainder of the stop.
In RABS "open" system, if there is a low fluid condition and the red warning light comes on, the ABS electronic control unit will detect this condition, turn the amber (Yellow) ABS warning light on and disable the system. The vehicle operator will still have basic brake system performance, but the benefit of RABS is lost.
The reason for disabling the RABS is that the low fluid condition could be caused by a leak in the brake system, decreasing its performance.
That amber (Yellow) ABS Light in Instr Panel should Light for about 2 seconds when you start the Bronco; If you never see it come on, check Bulb; many owners remove the bulb instead of troubleshooting & repairing system
Here is the Overview by Jem
