What I am curious about is: why the brake pedal sinks after 6-7 sec. of sustained pressure by my foot?

I also own a 1998 528i and 2 Volvos at home, these other cars don't behave like this (i.e. no pedal sinking with sustained braking on ice).

I wonder if the E53 ABS Hydraulic actuator has a "secret channel" where extra brake fluid is channeled into, creating a brake pedal sinking effect.

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