Master Cylinder Slime

Discussion in 'General Chevy & GM Tech Questions' started by Stretch56, Aug 9, 2014.

  1. Stretch56

    Stretch56 Member

    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.
  2. tbplus10

    tbplus10 Epic Member Staff Member 5+ Years 5000 Posts Platinum Contributor

    Do you tow much with your truck?
    I've seen slime like you described after brakes have been overheated.
    An annual brake system flush is the best way to take care of the system.
  3. Stretch56

    Stretch56 Member


    I have towed my trailer to Sequoia National Park quite a bit, climbing from 330' elevation, up to 7600' but I never worried about the brakes on descents because of the trailer brakes. I always downshifted on grades to save the brakes and never felt like they got to hot. With my 3/4 ton, 4:10 gears and my 8.1L, I always felt it was a good tow rig.

    The rotors all look good and I still have wear left on the pads. I know it's been a few years since I last had the fluid changed because of the cost, but I'll start doing it myself more often since I know the slime is there. It almost looks like mildew and coats the inside of the MC. I don't want to wipe it off the inside of the MC because I'm afraid of dislodging it. I don't know the brand of fluid that was last used, but I'm using synthetic now, so we'll see if it makes a difference.

    Thanks for the advice.
  4. RayVoy

    RayVoy Epic Member 5+ Years 5000 Posts

    The black slime is usually from corrosion within the braking system. Dot 3 and 4 fluid will absorb water and the combination will corrode the metal inside the system.

    Synthetic (Dot 5) fluid will not absorb water. However, it is not compatible with systems designed for dot 3/4. The result could be a system failure just when you need it most.
  5. Stretch56

    Stretch56 Member

    Thanks for the insight but I'm somewhat confused on the corrosion issue. The slime is up in the plastic
    fluid reservoir on top of the Master Cylinder, so did the slime radiate up from the MC? Or is it caused from condensation in the fluid reservoir?

    - - - Updated - - -

    In researching the slime issue further, I found that it could be algae, or a chemical reaction to minerals like Copper/Brass used in brake line washers and possibly Power Steering Fluid mixing with Brake Fluid because of a leaking Hydro Boost. The Hydro Boost piston leaked enough for the Master Cylinder Piston to pull some Power Steering Fluid back with it, apparently causing the contamination.

    Sounds far fetched, but I just replaced my Hydro Boost because it was leaking at the piston seal, so maybe that's just a coincidence.
  6. RayVoy

    RayVoy Epic Member 5+ Years 5000 Posts

    I forgot about the 2500 trucks having the hydro boast and a possible contamination issue. I was thinking about water contamination and the water/brake fluid combo causing corrosion within the full system.
  7. Stretch56

    Stretch56 Member

    I'm going to pull the reservoir off this weekend and clean it, just to be safe.

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