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?
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:
As you can tell, it gets darker as it gets older.
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).
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.
