About a week ago I bleed my brakes on my car, a '96 Honda Accord. On the right is the old, dirty, and pretty nasty brake fluid I had in my car. On left is brand new clear, clean, and new brake fluid, which is how all brake fluid should look. The difference in color is very clear.
The brake fluid I had in my car was obviously dirty, old, and black. Did you know old brake fluid is a safety issue? The reason is DOT 3 and DOT 4 brake fluid is hygroscopic, meaning it absorbs moisture. Think of brake fluid like a sponge. Overtime the fluid naturally absorbs moisture, which is why it must be changed. Moisture exceeding 2% is considered excessive, and brand new brake fluid has about 1.5%-1.65% moisture. Brake fluid is designed to have a high boiling point due to immense heat generated when braking. Moisture causes the normal brake fluid boiling point to drop, which can lead to brake fade or failure in extreme cases.
Contaminated brake fluid can also potentially affect the performance of your brakes and damage parts. All that dirt and moisture was moving through the brake lines, calipers, and wheel cylinders. When the dirty brake fluid was going through the ABS Actuator, delicate solenoids and motors can behave unpredictably. (My brakes work much better now that I flushed my brake system.)
It is easy to tell when your brake fluid is dirty, but how can you tell when you brake fluid has moisture? There are specialized strips to test the moisture content in your brake fluid, but no shop I have seen uses them. They will usually see people with brake fluid pictured on the right, and just recommend a brake flush. You could buy these strips in an auto parts store if you are curios to know.
Most manufacturers recommending flushing your system of old brake fluid and replacing it every 30,000 miles or 2 years. I think this is a good rule of thumb and should keep your vehicles brakes working well.
I thought this would serve as a good lesson. Hopefully you will change your brake fluid and not let it get this dirty!