That "soot" is just a sign of wear, it's from the rubber cups in the master cyl, a fine layer is actually harmless unless your fluid is really nasty. If your fluid is dark, coffee like colored, then flush your system. There isn't anything special to do with the ABS on these cars.
To flush it, what I do is I'll take an empty oil bottle, spray it clean with brake parts cleaner (best stuff in the world) then take a 2 foot clear line, one that fits the bleeder tight (found at lowes or hardware stores), cut a hole in the lid of the oil bottle then push the vacuum line all the way in to the bottom, "prime" the bottom of the bottle with about 1/2 inch of brake fluid, stand it up on the ground push the hose on the bleeder, crack it loose then just pump it with top to floor pumps until it's clear, just remember to refill the master cyl after the 4th or 5th pump and to close the bleeder before you pull off the line, then repeat at all 4 wheels.
Last edited by Starmavin; 08-18-2006 at 10:45 PM.