In other articles, I’ve discussed the importance of maintaining different fluids in your car. In this article, we’ll look at brake fluid, and I’ll reveal a little secret or two that the average person doesn’t know. So, let’s take a deep, but not literal, dive into brake fluid.

What is Brake Fluid?

Brake fluid is a type of hydraulic fluid that flows through the brake lines and calipers in the brake system of a vehicle. It is used to amplify and transfer pressure from your foot to the brake calipers or pistons to stop the vehicle. This is actually a pretty awesome feat since the kinetic energy of a vehicle that weighs several thousand pounds, moving at 40 or 50 miles per hour is amazingly great.

The fluid makes it possible to stop the vehicle with relative ease. Without it, braking would be extremely difficult, and in some cases, impossible. I know this first hand because I have driven a 1931 Model A Ford that has manual brakes and it is pretty scary. As important as brake fluid is, few people ever think about it. Gas, oil, and even radiator fluid are much more popular and well known to the average motorist. So the question is, do you really need to think about brake fluid? The simple answer is, YES!

Topping Off Brake Fluid

As the brake pads (or shoes on drum brakes) wear down, the level of the brake fluid naturally goes down. Not a lot, but some. As this happens, it is a good idea to add a little more brake fluid to the reservoir. Be sure to check your owner’s manual for the correct type of brake fluid and instructions on how to properly refill it. Brake fluid usually comes in DOT 3, DOT 4, or DOT 5.1. So, be sure to check which type your vehicle uses.

Brake Fluid Contamination

Brake fluid actually deteriorates over time. The cleaning and anti-corrosion agents that are added break down over time due to the contamination of water and copper in the brake lines. Water is less of a problem with modern vehicles than it used to be. The newer design of brake system parts and improvements in rubber hoses has virtually eliminated this problem. However, a loose cap and rain or water from a car wash can wreak havoc. Water in the system can corrode parts and cause the fluid to overheat since the boiling point of water is less than brake fluid.

Besides water, air can enter the lines and cause the brakes to feel soft or “mushy.”  This can be fixed by bleeding the brakes.

Yet another contaminant, and perhaps the worst, is copper.  Brake lines are actually lined with copper. Over time, the copper seeps into the fluid and can cause parts to corrode. You can test the amount of copper in your brake fluid using test strips available on If the amount of copper exceeds 200 ppm (parts per million), you need to flush and change your brake fluid.

Replacing Brake Fluid

So here is the fact that few people know: you should replace your brake fluid. Yes, it is true, you should flush and refill your brake fluid. Most people don’t. It is important to do this because contaminants can cause your brake fluid to fail. Believe me, I’ve had this happen. It is a horrible feeling when you press the brake pedal, your foot sinks to the floor, and nothing happens. Most repair shops or dealerships can test the brake fluid and flush it for you. However, I recommend that you ask them how many parts per million the copper level was. This will prove you know a thing or two about brake fluid. Again, if it is over 200 ppm, you should replace the brake fluid.


Next time you check your oil, radiator fluid, and other essential fluids, be sure to include a quick peek at the brake fluid reservoir. You’ll be happy you did.


Written by Gary Pradel