Last spring I replaced the slave cylinder and flexible hose, disassembled the master and cleaned all the gunk out. I also blew the hard line, put it all back together using Prestone synthetic Dot 4 fluid.

It has all worked perfectly since then but I just took the cap off the M/C and the fluid is BLACK.

I guess I know I'll have to buy a new rebuild kit or a whole new unit but I'm just trying to figure out what is happening, any ideas?

Thanks,

Chris

My first guess is that some rubber component somewhere is deteriorating.

Hi Chris,

A little secret. All DOT3, 4 and 5 brake fluid is synthetic. Its just a sales pitch.

As a general rule, DOT 4 has a higher boiling point at 230degC (new state without water absorbtion) vs DOT 3 which boils at 200degC. Brake fluid is hydroscopic and that reduces the boiling point a lot. It can reduce DOT 3 down to 130degC if it absorbs enough water.

They are glycol based and can not be mixed with DOT 5 which is silicone.

EDPM is the correct rubber to use with DOT 3 & 4. If the rubber is new and breaking down already. I would suspect the material used or the metal line was not clean. Are you sure all the rubber parts are EPDM?

I flushed my brakes and clutch in November before the roadworthy inspection and it was still clear after two years service.

I had the same problem with all new parts. I ended up replacing all three parts again over time and also considered the cap. I haven't checked recently to see if the issue is gone for good. I seem to recall that the master cylinder seal showed a small amount of wear which may have contributed the particles that caused the color, but I didn't find anything significant anywhere.

In the polymer world one way to make materials cheaper is to add filler. Some seal maker out there has probably loaded their formulation with carbon black and it's leaching out. That was my best guess at the time.

Which brand of parts are you using? For the record. Maybe we can identify the bad actor.

Zed, that's got to be it, the cap rubber seal did look a bit puckered but I didn't even consider it.

The M/C came with car and from the bills in the service record showed it was replaced in 2000 so I didn't see any reason to replace it. I have used Prestone Dot4 for many years now and never had a problem with it.

I'm going to pull the M/C and take it apart and try and see where these black particles are coming from.

Thanks,

Chris

If it was replaced in 2000 its getting to the point where EPDM can deteriorate.

Often called the "rate of degragation" in the rubber industry. It will then release small amounts of carbon black and other components and that is what your seeing in the fluid. EPDM can be sulphur cured or peroxide cured.

Peroxide is the better longer lasting method and the compound can stand more heat. The better brands use peroxide, but its more expensive. You pay for what you get, so they say.

Look for signs of wear on the cups and thats you cause.

Chas

It's the master cylinder, replace it with a new ( aftermarket ) one.. then refill with dot4.

Chas, thanks for all that info, interesting stuff.