Pics show as best I can. Brake fluid last changed March 2023 at 8500miles, mileage now approx. 14350…
Anyone seen this before?
Brake fluid is well known for being hygroscopic, so no surprise to find mould growing in old brake fluid.
Normally one would expect brake fluid to last two years. If old stock fluid is has been poorly stored, unsealed for example, it would not last so long.
I would invest in a brake fluid tester and see what the moisture content is before deciding how to proceed.
A thorough brake fluid flush before replacing the fluid would appear to be in order.
And the clutch!
Is that just black bits from the cap seal?
Take the strainer out clean it and the cap see how it is after.
I think I need a visit to Specsavers.
Thanks, no its around the filler neck and all around the inside of the plastic reservoir at the seal with the other half.
Yep sorry not that clear but definitely there.
Corrosion inhibitor breakdown, water in the fluid, copper from the brake lines?
Thorough brake system clean, new fluid etc…
Based on hypothesis rather than experience.
Does look like verdigris doesn’t it.
I’d suggest a piece of kitchen paper rolled up or scrunched, then dip it in and pull it out (carefully, don’t want to drip brake fluid everywhere) to get a better view of it.
Renewing brake fluid on most cars is straightforward enough. Rather than bleeding the whole reservoir full through the bleed screws, these days I use a meat baster (cheap one from a supermarket is perfect) to remove as much brake fluid as possible from the reservoir. I then top up the reservoir with fresh fluid and bleed the lines to each wheel. I use an “eezi-bleed” tube and bottle to collect the old fluid and it can then be done without an assistant.
(I did once ask my wife to pump the brake pedal for me without success. After ten very frustrating minutes of lying on my back and being unable to get and fluid to bleed out at any wheel I noticed the cable operated clutch arm was moving…she was pumping the clutch pedal… after that I went back to the one man method 
).
I wouldn’t be overly concerned about it. Clean it all up, give it a brake fluid change and go for a pint. It’s not unusual to see it.
Based on these pics the Mazda dealer has told me over email as follows:
“I have shown the photos to the technician, and he has informed me that it is normal to see a small amount of mould above the fluid line inside the top of the reservoir. This is due to the brake fluid being hygroscopic and the small amount of air in the top of the reservoir. Any mould present is usually cleaned off by the technician during a brake fluid change.
The technician has also said that if we were to test your brake fluid now, we would expect the moisture content to be around or below 1%. It is recommended brake fluid is changed every 2 years.
When we carry out a brake fluid change, we always use new brake fluid from unopened 1 litre or 500ml bottles. I believe there is also no copper in your brake lines.
We can book your in for a waiting appointment to check the moisture content of your brake fluid if you wish?”
I tested the fluid myself yesterday using an electronic tester, it shows <1% water content at this point.
The “mould” is not just around the filler neck but all around the inside edge much like a tide mark.
I guess I’ll see how it goes for now, next fluid change due next March.
Holy cow, I’m going with a 50/50 mix of brake fluid and bleach from now on, I don’t want my brakes looking like my seats!
