I'm a little unclear about the nature of the sound. On the heading it says "ABS chatter" but in the body of the text you describe it as "grinding." I don't know what would likely cause a grinding noise. On the other hand, if you're talking about feeling brake pedal pulsations at low speed, esp. with wet rotors, from the activation of the ABS, you may just need to clean out your wheel sensors. Lots of posts on this so do a search and you'll see what you need to do.

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Steve
'93 400E