Topic: Why is my brake fluid always getting dirty?

[Buttmask]

For starters:

'75 750F0
Rebuilt, leak-free master cylinder, clean as a whistle when starting
Stainless lines from Slingshot, new copper crush washers on all banjos
Rebuilt, new seals, fairly pit free piston caliper
Honda OEM pads
Bleeder valve

After a couple weeks with crystal-clear brake fluid, I always start getting brake squeal and glazed caliper on the piston side of my disc, open up the master and my brake fluid is black.

The rubber seal is good, and gets tightened down well.

Any ideas?

[HondaMan]

For starters:

'75 750F0
Rebuilt, leak-free master cylinder, clean as a whistle when starting
Stainless lines from Slingshot, new copper crush washers on all banjos
Rebuilt, new seals, fairly pit free piston caliper
Honda OEM pads
Bleeder valve

After a couple weeks with crystal-clear brake fluid, I always start getting brake squeal and glazed caliper on the piston side of my disc, open up the master and my brake fluid is black.

The rubber seal is good, and gets tightened down well.

Any ideas?

Are you using DOT4 or DOT5 fluid? If so, it is dissolving the new mater cylinder seals (and brake caliper O-ring) that you installed. These are all Buna material, and the synthetic DOT4/5 formula dissolves it. They need to live in DOT3 or J1703 type fluid.

[Buttmask]

For starters:

'75 750F0
Rebuilt, leak-free master cylinder, clean as a whistle when starting
Stainless lines from Slingshot, new copper crush washers on all banjos
Rebuilt, new seals, fairly pit free piston caliper
Honda OEM pads
Bleeder valve

After a couple weeks with crystal-clear brake fluid, I always start getting brake squeal and glazed caliper on the piston side of my disc, open up the master and my brake fluid is black.

The rubber seal is good, and gets tightened down well.

Any ideas?

Are you using DOT4 or DOT5 fluid? If so, it is dissolving the new mater cylinder seals (and brake caliper O-ring) that you installed. These are all Buna material, and the synthetic DOT4/5 formula dissolves it. They need to live in DOT3 or J1703 type fluid.

I'm using DOT3, but it's labeled as synthetic.

I should note, that I don't have this issue on my rear brake, just the front.

[jeffg]

did you clean behind the caliper piston? When I took out my piston it was full of corrosion.

[Buttmask]

did you clean behind the caliper piston? When I took out my piston it was full of corrosion.

Yes, when I rebuilt the caliper, the whole thing got well cleaned and I lightly removed all the corrosion and pitting on the piston.

[HondaMan]

Hmmm...next question: are the brake hoses new, or used? If used, and formerly used with DOT3 but now with synthetic, same issue applies: the liner can dissolve. To stop this problem, I have lately been using Goodrich lines, which improve braking performance by not flexing (but can leave you able to lock the front wheel, if you have dual discs!).

[dusterdude]

All dot3 made in the last few years is synthetic but its still good ole dot3.dot5 is what you have to be careful of,as far as mixing is concerned

[jeffg]

I flushed about 3/4 of a bottle through mine to get it to run clear & clean.

I haven't checked mine since then, now you got me wanting check mine as well.

[Buttmask]

Hmmm...next question: are the brake hoses new, or used? If used, and formerly used with DOT3 but now with synthetic, same issue applies: the liner can dissolve. To stop this problem, I have lately been using Goodrich lines, which improve braking performance by not flexing (but can leave you able to lock the front wheel, if you have dual discs!).

Yep, brand new lines, have never seen a drop of anything but DOT3. I have a rear disc, same stainless lines, rebuilt everything, but that one stays clean. The front always black as night.

[Muckinfuss]

go to napa or autozone etc. and buy some brake fluid test strips.  in some set ups, moisture is the problem and the test strip will tell you that.  if you are getting moisture in such short time lines....then your master has most likely got an air leak in the cap.  if no moisture, then there's a remote possibility of copper dissolving into the fluid.  Usually this takes time, but it does happen. I'm not saying these are your most likely problems, but they are the easiest to eliminate with test strips.

If you eliminate those two as culprits, you might have, as others have said, crap seals/O rings in the system somewhere.  It would only take one.

edit:   once upon a time, GM issued a tech bulletin re: black brake fluid in Corvettes.  They stated that the Dot3 and Dot4 fluids had a behavior of leaching dark from the rubber components in the system but it wasn't an issue regarding brake system performance.  Also that certain fluids labeled high performance or race type, still Dot 3/4 had chemicals in them that accelerated the leaching.  Motul being one such brand. 

Since you don't indicate the rear was rebuilt other than pads and line....I'm thinking the most likely issue here is that the rubber rebuild parts for your front brake system are your culprit.

[doobiebro]

Would excessive heat build up from a dragging/sticking brake cause the fluid to turn black?

[HondaMan]

Hmmm...next question: are the brake hoses new, or used? If used, and formerly used with DOT3 but now with synthetic, same issue applies: the liner can dissolve. To stop this problem, I have lately been using Goodrich lines, which improve braking performance by not flexing (but can leave you able to lock the front wheel, if you have dual discs!).

Yep, brand new lines, have never seen a drop of anything but DOT3. I have a rear disc, same stainless lines, rebuilt everything, but that one stays clean. The front always black as night.

Here's my DOT3 choices. Note that one of them (the single bottle picture, NOT my choice for these bikes) is marked "synthetic" and it also does NOT have the J1703 compatibility note on it. I only use the DOT3 fluids that also list this specific note, because this was Honda's spec before DOT3 came out, and in the 1970s-80s the DOT3 always listed its matchup with the J1703 spec.

[Mr.Moose]

The local vintage japaneese bike store sold me a DOT5,1 and said its all they use in all bikes. Been in there for a week. Would you guys replace it With DOT3?

[flatlander]

i've been using motorex dot 5.1 all of this year without problems. i put it in a rebuilt and clean system.

[Buttmask]

Hmmm...next question: are the brake hoses new, or used? If used, and formerly used with DOT3 but now with synthetic, same issue applies: the liner can dissolve. To stop this problem, I have lately been using Goodrich lines, which improve braking performance by not flexing (but can leave you able to lock the front wheel, if you have dual discs!).

Yep, brand new lines, have never seen a drop of anything but DOT3. I have a rear disc, same stainless lines, rebuilt everything, but that one stays clean. The front always black as night.

Here's my DOT3 choices. Note that one of them (the single bottle picture, NOT my choice for these bikes) is marked "synthetic" and it also does NOT have the J1703 compatibility note on it. I only use the DOT3 fluids that also list this specific note, because this was Honda's spec before DOT3 came out, and in the 1970s-80s the DOT3 always listed its matchup with the J1703 spec.

Very interesting, the one you have listed as the one you don't use is definitely the exact one I've been using. I'll go ahead and give the generic O'reilly's one a whirl and see if that helps. Still weird though it has no effect on my rear brake system. Both front and back are both the same stainless lines, and both have Honda OEM piston seals and master cylinder kits.

[calj737]

The local vintage japaneese bike store sold me a DOT5,1 and said its all they use in all bikes. Been in there for a week. Would you guys replace it With DOT3?

As long as it's installed in a completely flushed and fresh system, it's usable. NEVER mix it with DOT3/4

[DaveBarbier]

The local vintage japaneese bike store sold me a DOT5,1 and said its all they use in all bikes. Been in there for a week. Would you guys replace it With DOT3?

As long as it's installed in a completely flushed and fresh system, it's usable. NEVER mix it with DOT3/4

There's a difference between DOT 5 and DOT 5.1. DOT 5.1 is glycol based like DOT 3/4 and the three can be mixed. DOT 5 is a different animal which is silicone based which can't be mixed with any of the others. Weird they named it DOT 5.1, though. It's more like a DOT4 with a higher boiling point. Confusing.

[calj737]

sorry, missed the x.1. Misread his comma. Never mind!

[RAFster122s]

Did you buy the caliper seals from Honda or are they aftermarket seals?  It could be possible that an old seal was mixed in with the other seals and that seal doesn't like the fluid.  I have had the fluid get dark on a car that was very old (Volvo 240) when using Castrol GT LMA fuild. I merely took it as a sign it was timeto bleed the brakes & flush the system.  It had been less than a year since the fluid had been changed and brake performance was good. It didn't get dark faster after that but would over the course of the year get darker. Bleeding brakes on a dual triangulated brake system like the 240 wasn't hard with a power bleeder (Ezebleed or Motive power bleeder) just time consuming to get the car on jack stand & wheels off to go through the 8 bleed points on the front and rear calipers in the proper sequence.
What brand of fluid are you using and was it a new bottle of fluid used on the front system?
Thanks, David