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Black Gunk in Break Fluid

I bled my breaks tonight and there was a bunch of black gunk in the break fluid reservoir. Also a little bit of it came out at the wheels. The fluid was dark red so but the gunk was pure black. The breaks haven’t been bled in about 30,000 miles, or 4 years. Is this a sign of something breaking down or some other looming catastrophic event? Thanks.

I don’t know the cause, but it’s not good. Brake fluid needs to be pristine, not mixed with black gunk. Potential sources for the gunk include a degraded rubber seal on the fluid reservoir or incorrect brake fluid used in the past. Even if you aren’t experiencing problems braking, left alone the gunk may cause the brakes to fail at some point. Have the fluid changed and the master cylinder bled along with the brakes, and the system tested and inspected by a professional.

That is definitely not normal. Normally cars go way longer and farther without having “gunk” in the brake fluid. If you haven’t already, take the reservoir off the master cylinder and clean it out, then bleed the system until all the fluid is completely clean. There isn’t any way to really tell if there is something bad looming on the horizon. Keep an eye out for leaks throughout the system. Fortunately if something is screwed up, a sudden and complete brake failure is extraordinarily unlikely, just watch for the signs.

Age and contamination can cause black sludge but why is the fluid red?

  1. The black gunk is possibly (probably?) from deteriorating inner linings of the flexible brake hoses. It’s not uncommon for them to start to come apart after 10-15 years. The typical failure seems to be that the inner linings eventually detach from the tough outer walls and hold the brakes on after you apply them. If you have the skills to bleed the brakes, you probably have the skills to replace the hoses. They usually aren’t terribly expensive.

  2. I haven’t seen all the brake fluid in the world, but all the brake fluid I’ve ever seen was clear with a slight yellowish tinge. Unless someone can tell you that dark red is a normal color for GM brake fluid, I’d get that stuff out of there and replace it all with fresh fluid … after replacing the flexible hoses if you are going to replace them.

I believe that vtcodger is correct.
The black gunk is very likely the result of badly deteriorated flexible brake lines. I would strongly suggest not driving the car until you replace all of the lines. It is very possible that whatever strange red fluid was put into the brake system led to the deterioration of the flex lines.

“The fluid was dark red…”?

I suspect someone put transmission fluid in the brake lines. This feels like a very dangerous situation. Change it out quickly, and replace the flex lines, like vtcodger2 mentioned. Since the flex lines need to be replaced, flushing out the calipers and wheel cylinders should also be considered.

I agree the “dark red” fluid is very odd and suspicious.

When was the last time these brakes were bled or the last time brake fluid was added? I’m wondering if there was any chance a petroleum based fluid was recently added.

I check my fluids at the very least once a month and sometimes weekly. I have added break fluid as I have needed it, but always dot 3 fluid from autozone (which is an amber hue). I got the car with my wife :slight_smile: about 3 years ago and have done all my own repairs and maintenance since then. She had replaced her breaks about a year before and I assumed they had replaced the fluid, since I always do when I change my breaks. I just replaced the breaks and so after that I put all new fluid in, last night that is when I saw the gunk. I will admit the dark red old fluid seamed weird to me but I thought it was just dirty. I will take your suggestions and replace the hoses as well as re-drain the system and take off the reservoir and do a better job than just sucking out the fluid. I hope I didn’t idiotically put transmission fluid in there some time in the past, I don’t think I did but who knows. I did a search online and did find a couple of references to red Dot 3 break fluid, so hopefully it was just that. Thanks so much, off to the parts store!

Possibly the rubber boot under the master cylinder cover was being emulsified by an errant fluid. Was that boot unusually soft and swollen?

I don’t think I have a rubber foot under the master cylinder. Also The only hoses that I have are from the wheel well to the caliper assembly, approx 1 foot per wheel. Everything else is metal tube. I don’t see how that would have put so much gunk in the reservoir. My suspicion is that the seals in the master cylinder have corroded since that is the only rubber or plastic other than the reservoir and the hose at each wheel. Does anyone think I need to overhaul my master cylinder?

Does anyone think I need to overhaul my master cylinder?

If you have any inclination that a petroleum-based fluid found its way into the system, then you’ll be strongly advised to replace or rebuild the master cylinder, calipers and rubber flex hoses.

If the ABS unit has rubber components in it, you’ll need to address that as well. But I’ll let those with more experience on ABS internals comment on that.

Is there any difference between overhauling my master cylinder (25 dollar kit, NAPA) and buying a refurbished model (55 dollars, NAPA)? A new one is 145 so I would prefer going one of the other routes. Thanks.