You have probably have heard and also been given this recommendation by your auto repair professional. “Your brake fluid is black and needs to be flushed” and you usually decline because you feel that your vehicle is braking and stopping just fine. That may be true, but overtime many factors will begin to affect your braking system.
The standard Dot-4 brake hydraulic fluid that Toyota, Lexus, and Scion and most other manufacturers use is hygroscopic-that is to say, it actively absorbs moisture out of the air, which then becomes a corrosive contaminant. Toyota is silent on service intervals for the brake hydraulics, which is odd, because Lexus-their sister company-recommends that it be flushed every 30,000 miles, which seems to be a common (though not universal) industry standard elsewhere as well.
The brake fluid starts out clear and almost completely without color. It gets darker and transitions to amber and eventually black with age. In the absence of service records, this serves as a casual indicator of age. Just as a casual aside, the clutch hydraulic fluid, which is identical to the brake fluid, turns black much more quickly than the brake fluid does. I speculate that this is perhaps because of two factors:
First, the two systems are each exposed to the air through vent caps with the same sized vents in the reservoir caps. The clutch hydraulic system though has a much smaller amount of fluid in it. As a result, the same total amount of moisture absorbed becomes a much higher percentage of the whole, and so has a greater effect.
Secondly, the hydraulic fluid in the brake system is routinely exposed to much higher temperatures, and so has a tendency to cook some of the moisture back out of the fluid. This is just my theory, and I haven’t ever seen it discussed anywhere else.