OK,

Today it was snowing and icy. During a hard brake at 20mph (no accident or anything, just going down the hill) with foot firm on brake pedal, the ABS was activated and about 6-7 sec later the pedal goes down further, basically only 2-3 inches from the floor.

On dry road, there is no brake pedal sinking.

Are there some valves in the ABS Hydraulic Module that allow fluid to "flow around"?

I know the Master Cylinder is good b/c on dry road, the brake works just fine.

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1998 E39 528i 5sp MT
2006 E53 X5 3.0 6sp MT