I just replaced the brake fluid in my 03 Yukon XL 2500 today, and found a slimy black residue in the Master Cylinder. I sucked the old fluid out of the MC before replacing it with new and then did an ABS vacuum style bleed. I found some of the same residue in the RR brake line during the bleed, so I did another compete bleed on all four until the fluid was clear. I know that any dirt or foreign material in the MC can be a problem, so is the slime normal signs of fluid breakdown? And if so, is there another remedy besides replacing the MC or should I not worry about it.