How easy is it to contaminate brake-fluid in a car, what is the consequence, and what is the correct way to clean it?

A mechanic just changed my brake-fluid. He was sloppy and did not clean the small plastic filter-mesh that goes into the top of the brake-fluid reservoir, so it had some black oil and dirt on it, not a lot, but it was definitely dirty.

I have read that the brake-fluid must be 100% clean otherwise it can give serious problems with the brake-system. It even says so on the lid to the brake-fluid reservoir - which the mechanic had also covered in black oil so I had to clean the lid to read the text.

I have researched for hours on the internet how much oil and dirt is enough to contaminate the brake-fluid but I cannot find a clear answer. Some people say that some chemicals can contaminate the brake-fluid with just a few drops, but it can be a long time (how long?) before the damage actually results in the brake-system failing. If you're lucky enough to discover the problem before you crash the car, it would still be very expensive to replace the entire brake-system.

So I demanded the mechanic replace the brake-fluid and clean the reservoir. That took him more than 2 hours. But for some reason he didn't bleed all the fluid out of the brakes, he just disconnected the reservoir, cleaned it, and filled it with new fluid.

I don't know what he used to clean the reservoir, but I did see him spray strong brake-cleaner on both sides of the top-lid itself, so my guess would be that he maybe used some kind of strong liquid degreaser to clean the inside of the reservoir?

Considering how sloppy he was in the first place, I am also concerned if he might have used the wrong chemicals to clean the brake-fluid reservoir. How sensitive is the brake-system to that?

Since he didn't bleed the brakes when cleaning the reservoir, I suppose there could still be contaminants in the brake-fluid inside the brake-lines? So I guess I will have to get another mechanic to bleed the whole brake-system again to make sure it is 100% clean? Would it be a good idea to maybe fill it up with brake-fluid and bleed it an extra time to get all the contaminants out of the system? Brake-fluid is fairly cheap so it's just the work-hours that'll cost.

My car is a Fiat Ducato 2004 with about 0.6 Liter DOT-4 brake-fluid.

1 Answer 1


Given your other question, which I answered, I would push fluid back from the calipers into the master cylinder so no contaminants penetrate any further into the system. And then empty and refill from scratch.

Hydraulic systems work on very small tolerances and in other hydraulic systems - level control etc “new engine oil” is classed as dirty ... believe it or not ...

  • Thanks for your suggestion. I'm glad to hear that I wasn't being hysterical when I demanded the brake-system be cleaned. But are you sure it's a good idea to push the fluid back from the calipers because that is where the "maybe contaminated" brake-fluid still is, while the reservoir should be clean now with new and clean brake-fluid (unless of course he used the wrong cleaning agents that have now polluted the reservoir)? May 19, 2018 at 17:11

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