Why does brake fluid turn dark when it seems clean?

Dear Car Talk

Dear Car Talk | Oct 01, 1996

Dear Tom and Ray:

Could you please clear up a question that has been nagging this weekend
mechanic. Why are so many brake master cylinders mounted at such odd
angles? My older cars have nice, level reservoirs, and they seem to brake
just fine. Also, why does brake fluid turn dark even when it seems clean?
Does the fluid ever need to be flushed and replaced? -- Gary

RAY: The answer to your first question is space, Gary. As cars have gotten
smaller, more things get crammed into smaller engine compartments. And the
brake master cylinder is one of those things that can go almost anywhere --
so it does.

TOM: And the angle doesn't make any difference. As long as the fluid is at
the recommended level, it works just fine, which is why your newer cars
stop just fine, too.

RAY: And yes, you should flush your brake fluid regularly. The rubber seals
in the master cylinder, the wheel cylinders and calipers deteriorate over
time. And that deteriorated rubber is what makes your nice, clean brake
fluid turn dark.

TOM: We suggest a brake-fluid change every 30,000 miles. And if the angle
of the master cylinder continues to bother you, try tilting your head
slightly when you look at it.

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