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I've got spongy brake feel.

This situation has me wondering if it is at all possible to tell that brake fluid needs changing just by looking at the fluid in the reservoir, similar to how you can tell if you need an engine oil change if the oil is black.

Does moisture ingression affect the appearance of brake fluid?

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Yes. When brand new, brake fluid looks clear. Once there is a significant amount of water absorbed, it will turn an amber color. This applies to regular brake fluid (DOT 3, 4, & 5.1) and not synthetic. Here is an image of new and old brake fluid:

enter image description here

As you can tell, it gets darker as it gets older.

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  • 2
    For some reason I always thought brake fluid was brown when brand new. The things you learn every day... :) – Zaid Sep 20 '15 at 14:32
  • Hmm, I'm not sure about this. I opened a sealed can of DOT4 several years ago, and the liquid was already amber. Those days, I again opened a sealed can of DOT4 (other brand), produced in February, and it's also amber. I can imagine it becomes darker over time, but I have never seen brake fluid as clear as water. – sweber Oct 6 '15 at 13:21
  • @sweber - I'm sure different manufacturers can do it however they want. Basically, the color will go from (relatively) clear to murky (but still transparent) to dark and you just cannot see anything through it. – Pᴀᴜʟsᴛᴇʀ2 Oct 6 '15 at 15:11
  • @Paulster2: I think so, too. But this picture is suitable to scam drivers, as they all see that amber liquid in their cars compared to this water clear liquid on the photo. They'll change it, though not necessary. (And depending on what brand they get in, claim that they got old liquid...). Finally, it's worth to note that new liquid isn't always as clear as water. – sweber Oct 6 '15 at 19:33
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Rather than rely on looking, you can purchase a brake fluid tester which works on a single AA battery and dips into the fluid. This will instantly tell you how much water has been absorbed by the fluid and therefore if it's still serviceable.

Considering how vital your vehicles brakes are, I'd suggest that it's worth the investment (they are fairly inexpensive).

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Like most automotive chemicals, there are corrosion inhibitors in brake fluid. These inhibitors wear out over time. The state of these inhibitors can be measured with test strip. The strip measures copper levels. Copper levels in the brake fluid are an indicator that the fluid’s corrosion inhibitors are depleted. Copper levels can predict when corrosion is occurring. The cut off of acceptable copper levels is 200ppm. Copper can plate to ABS and other valves causing inconsistent ABS operation. The source of the copper is the steel brake lines according to Phoenix Systems a test strip supplier. I assume stainless steel lines would not be subject to corrosion. The calipers, master cylinder and any other conventional steel parts would still be subject to some corrosion but there is no mention of them releasing copper.

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