Psfred is only right in the first part of his explanation.
ABS will default simple indeed but with a "normal" failure.
The symptoms you are discribing are NOT normal.
Indeed, ABS releases pressure in the brake of the slipping wheel.
But where does that pressure go you think ?
Not into the brakefluid reservoir but into the master brake cylinder.
Why ? To keep your brakepedal HARD ! Otherwise you would feel the pressure releas AND you would compensate it by pushing harder on the pedal !
And as the pressure release is rapidly pulsing, you get that odd feeling in your right foot as a warning that there is an intervention of your ABS and thus your car is loosing "grip".
So your problem looks very scary to me ! I never opened up an ABS so you'll
need a rebuild or a new one. Most certainly not cheap at MB's stealership !