Brake fluid quietly degrades and picks up contamination over years, and bad fluid undermines everything the brakes do. Knowing the signs lets you catch it before it causes a soft pedal or damage. Here is what contaminated brake fluid looks like.
Dark color
Fresh brake fluid is clear to light amber. As it ages and absorbs moisture and picks up rust and rubber particles, it darkens to brown or nearly black - see brake fluid color meaning. Coffee-colored fluid in the reservoir is a clear sign it is contaminated and overdue for a flush.
Moisture contamination
Standard brake fluid absorbs water from the air over time, which lowers its boiling point and invites internal corrosion - see brake fluid boiling point. You cannot see moisture, but a test strip measures it. High moisture is the most common contamination and the reason for time-based flush intervals - see how often to change brake fluid.
Wrong fluid or oil contamination
Mixing incompatible fluids - like silicone DOT 5 into a glycol system - ruins it; see brake fluid compatibility. Worse is petroleum contamination (motor oil, power steering fluid) getting into the brake fluid, which swells and destroys the rubber seals throughout the system - a serious problem requiring major repair.
What to do
Dark or moisture-laden fluid should be flushed and replaced with the correct fresh fluid - see brake fluid flush guide. If petroleum contamination is suspected (swollen seals, a mushy pedal soon after a fluid service), the system needs seals replaced. Keep fluid fresh alongside quality pads and hardware.