Not exactly sure but it has to allow for the ABS system to bleed the hydraulic pressure off the brake slave cylinders/pistons when the wheels stop turning. As long as the wheels don't lock the pressure is kept steadily applied. Once they lock, for a few milliseconds, the pressure is released and the wheels begin to turn again. Since the operator still is pressing on the brake pedal, pressure is routed back to the system. And the cycle continues until the car is stopped or one takes their foot off the brake.
There has to be some difference in the MC. I presume excess fluid is ported back to the reservoir.
JR
