New brake fluid turning black.

[rob07]

I've recently put in a new brake master cylinder in my car and in the both reservoirs (front and rear circuits) the brake fluid has gone almost black after a few weeks. I had flushed god knows how many litres of brake fluid through the system in trying to previously resolve the problem before putting in the new master cylinder and booster with new seals. At the same time I also put in a new clutch master cylinder and obviously bled everything with new dot 4 fluid. The fluid in the clutch reservoir has remained clear unlike that of the brakes.
I can only guess the seals in the brake master cylinder are dissolving, but I did soak them in brake fluid for a couple of days first to check compatibility and they were fine.

Edited by rob07 on Saturday 14th March 03:48

[Richard-D]

I was thinking seal or flexi hose compatibility as soon as I read the title but have never encountered that issue in practice. It does sound likely that the seals you've fitted may not be resistant to brake fluid. Do you know what material they are?

[rob07]

The master cylinder was a top quality Italian made item and had rubber seals. But when the fluid turned black the first time, I changed the brake master seals for some which I had previously bought but never used for the worn original master cylinder. These came from Pastparts which are a respected brake reconditioners and they insisted that these were quality items which have never had issues.
So for two separately sourced seals to do the same thing seems extremely doubtful. Also it can't be a corrosive brake fluid as there were no issues with the clutch master seals.
I can't see it as being the booster as this only pushes a metal rod against the brake master piston and is separate from the brake fluid run.

[steveo3002]

dont assume pastparts bits are premium , i paid strong money to get a slave rebuilt , it failed in less than a year , then they rebuilt it but suggest it was bad fluid and the rebuilt one failed again

yes thats right , i pay nearly £100 to get it rebuilt but reused some pond water instead of £4 worth of fluid

[Dynion Araf Uchaf]

You don’t say whether your car has anti locking brakes and therefore an abs pump. If it does have you activated the pump during the bleed process via a scan tool or similar?

[E-bmw]

Richard-D said:

I was thinking seal or flexi hose compatibility as soon as I read the title but have never encountered that issue in practice. It does sound likely that the seals you've fitted may not be resistant to brake fluid. Do you know what material they are?

I too thought this, but have never come across it in real life.

OP.
When you say the fluid was black, was that at the reservoir or you noticed it when bleeding fluid out, so at the calliper ends?

[rob07]

No anti lock brakes here. I've yet to bleed the system after seeing the dark fluid in the reservoirs this second time but I'm confident the fluid coming out of the calipers will be clear until the dark fluid in the reservoirs comes out.
I rebuilt the calipers front and rear a while back and they have never given any trouble since.

[E-bmw]

OK, the next obvious question is how "black" is it?

I ask because with non-silicone based fluids when they become moisture contaminated the colour progressively goes darker, it can easily be described as various colours of brown but I (personally) wouldn't call it black.

If it is becoming darker at the reservoir end I would be trying to find if there is ANY way water could be getting in there.

[paul_c123]

Pics of fluid in reservoirs would be useful.

[rob07]

First image is the fluid in the reservoir,visible below the zinc retaining ring, second image is the fluid in a small jam jar.

[rob07]

[E-bmw]

I am pretty sure that is water contamination.

How long did it take to go that colour?

Do you have a baked bean can or similar to hand?

I would be warming it gently on the hob to see if water vapour comes off it for confirmation.

[Elowen Ray]

Hey, I read your post and thought this information might be useful to you.
I only noticed recently that AUTODOC MARKETPLACE is a thing now. Looks like they’re turning their catalogue into more of a marketplace with different sellers. Haven’t ordered from it yet, but the selection already seems bigger than before.

[E-bmw]

Elowen Ray said:

Hey, I read your post and thought this information might be useful to you.
I only noticed recently that AUTODOC MARKETPLACE is a thing now. Looks like they re turning their catalogue into more of a marketplace with different sellers. Haven t ordered from it yet, but the selection already seems bigger than before.

I am calling robot!