Brake fluid is black or brown

Your car requires several different fluids in order to operate. Engine oil, transmission fluid, power steering fluid and coolant are all essential. Another important one is brake fluid. Like other fluids, your brake fluid will deteriorate over time and eventually need to be replaced with a brake system flush. If you’ve noticed that your brake fluid appears black or brown, it’s well past time for a service.

How this system works:

Your brakes are operated by fluid pressure. The master cylinder and brake booster work to create pressure, which is then directed to the moving parts of the system (the calipers and shoe actuators). In a normal system, the brake fluid is clear or very light gold in color. This is true whether your car requires DOT 3, 4 or 5 brake fluid.

Brake fluid is held in the master cylinder reservoir, but it is also stored in the brake lines that run from the master cylinder to each wheel. You have two types of brake lines – steel and rubber. Over time and through normal use, brake fluid becomes contaminated. This happens on all vehicles, even brand new ones.

It requires only a couple of years for the fluid to build up enough contaminants that the color begins to change. It will darken over time, changing from clear/golden to a darker gold. In severe situations, your brake fluid may appear to be brown, or even black (think used engine oil).

It’s important that your brake fluid is changed regularly, before it turns brown or black. Very dark colors indicate that your fluid has collected a significant amount of contamination, and might have absorbed moisture, as well. This reduces the system’s ability to build pressure, which compromises your braking performance. You may have noticed that your brake pedal feels softer, or spongy. In serious situations, you may have to pump the pedal a couple of times for it to work properly.

Common reasons for this to happen:

  • Normal Aging Gone to the Extreme: The most common reason that brake fluid appears brown or black is that normal aging has gone unchecked (you haven’t had the fluid changed in too long). Contaminants collect in the fluid, darkening the color and reducing its ability to work.
  • Moisture Contamination: Brake fluid can absorb moisture from the surrounding air. This generally occurs by rubber brake lines allowing moisture to seep in. When moisture accumulates in brake fluid, it causes problems in a couple of ways. Water has a much lower boiling point than brake fluid, and can reach very high temperatures at the calipers and drums. This causes the water to flash boil into steam, causing the brake fluid to darken, but also creating air in the lines. This can cause a soft pedal, or even cause you to have to pump the brake pedal to get the system to work properly.
  • Deteriorating Rubber Lines: The rubber brake lines on your car will eventually deteriorate and need to be replaced. Brake fluid is also corrosive, and while the lines are designed to withstand that corrosion, they’ll eventually begin to break down. Microscopic pieces of rubber and chemicals leaching out of the lines into the fluid will also cause it to darken.
  • Normal Heating: Brake fluid is heated by contact with the calipers, and this heat creates a chemical change in the fluid, adding to the darkening created by moisture and other types of contamination.

How important is this service?

Brake fluid darkens as it ages – it’s natural, and due to normal contamination that occurs in all brake systems. However, it must be changed regularly, or you risk reduced braking performance and the deterioration or damage of other brake system components.