My truck is experiencing the same thing. I tested it out in the rain a couple weeks ago and when I stop hard enough for the ABS to kick in my pedal begins to jump up and down so far my foot almost comes off the pedal (~1"-2"). I know you are supposed to feel vibration through the pedal from the ABS, but this is insane! Also I hear some rather loud metal clanking when the ABS is enganged. All other braking functions are fine and at no other time to I have the brake pedal activity.

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