Brake Fluid Sediment?

[Redsand187]

This is something I have been curious about for quite some time now but never really took the time to do the research.

I am a Honda Technician. One of our big selling services is a Brake Flush, based on time (36 month factory recommendation) and the brake strip testing. Pretty straight forward. However, I have noticed on most vehicles, the master cylinder has some type of sediment sometimes almost sludge like at least in the strainer, if not in the master cylinder it's self.

I have always been curious as to what this is, and what causes it. I would hate to be in a position were I can't answer a customers question, if it ever came up. I am unsure if it is just dirt getting in through the caps, and possibly through the calipers (which seems very unlikely unless all these vehicles have damaged dust boots on the caliper pistons) or if it is caused by a chemical reaction or something else.

Anyway's, I'd greatly appreciate any comments or answers. I did read that article that was posted a month or so ago on here, but it didn't really address any sediment build up.

 

[Nyquist]

I've always thought it was a combination of some oxidation over time and some trace amounts of moisture. I'm not sure though.

 

[Quest]

Most of those dark sediments are nothing short of very fine worn rubber "dust" as the seal rubs against the pistons/bores.

yes, contamination may have contribute to changes in colour also but not as significant as rubber wear.

Q.

 

[HTSS_TR]

Sediment caused partially by rubber cup on the reservoir cap.

 

[Boomer]

The fluid is fundamentally a polyethylene glygol liquid. It is hygroscopic. If the terminal groups on the glycol are OH groups, the can, over time, be oxidized to carboxylic acids, much like wine (ethanol) converts to vinegar (acetic acid). However, this would be a slow process and unlikely to be a major cause of the sediment. I agree with the folks above. You are most likely seeing rubber degradation products. And remeber, rubber is filled with MANY added chemicals to cure it. These can extract slowly and react to add to this "gunk."

 

[Big Jim]

Here is a recent thread on the topic that I think you will find very interesting.

http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=1272410#Post1272410

 

[mechtech2]

Seals

 

[artificialist]

I have seen that junk several times. What really bothers me is that now matter how thoroughly I flush, the stain does not go away.

 

[Kestas]

I try to agitate it to put the sediment in suspension when I suck out the fluid before bleeding.

 

[mechtech2]

arti - The sediment/stain on the reservoir walls has to be manually cleaned.
A reservoir removal may be necessary.
It is still a much better situation with fresh fluid and not perfect interior wall deposits.