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  • Exposure to water and air, solid particles, and rust can contaminate your vehicle’s brake fluid.
  • Your car’s braking system is a closed system that keeps contaminants out, but heavy braking, leaks in the brake system, and brake system maintenance can cause contaminants to enter the system.
  • An illuminated anti-lock braking system light, discolored brake fluid, and brake fluid leaks are common symptoms of contaminated brake fluid.

Like engine oil and transmission fluid, brake fluid breaks down over time. However, the hydraulic fluid in the brake system will degrade faster when exposed to contaminants like air, dirt, and water. Dirty brake fluid will reduce the brake’s performance, making your vehicle less safe to drive.

Fortunately, you can take steps to prevent the causes of contaminated brake fluid. And if you do end up with dirty brake fluid, you can fix the problem and restore your car’s braking performance. However, preventive and corrective measures require you to know about the issue, its causes, and its symptoms.

What Causes Contaminated Brake Fluid?

There are several possible factors that can lead to contaminated brake fluid. Here are some of them:

Exposure to Water

Water contamination is one of the most common reasons for dirty brake fluid. Most brake fluid formulas are hygroscopic. A hygroscopic fluid attracts water, whether it’s vapor suspended in the air or water in the liquid phase.

Water degrades the molecular structure of brake fluid. It lowers the fluid’s boiling point, which can reduce the brakes’ performance.

Furthermore, water causes corrosion in steel, especially if the metal loses its protection against rust. Corroded parts can shed rust flakes, which are considered a contaminant as well.

, Dirty Brake Fluid: Causes and Fixes

Pro Tips are nuggets of information direct from ASE-certified automobile technicians working with, which may include unique, personal insights based on their years of experience working in the automotive industry. These can help you make more informed decisions about your car.

Pro Tip: Brake fluid containers left uncapped can absorb water before even being added to the system. If the master cylinder reservoir cap is left off, the same thing can happen because brake fluid actually absorbs water.

Exposure to Air

Air and brake fluid don’t mix well together. The oxygen in the air reacts with the hydraulic fluid in a chemical process called oxidation. Oxidation lowers the fluid’s boiling point, which increases the chances of overheating and causes other brake problems. Oxygen also causes corrosion in brake parts. When it oxidizes steel, it creates rust, another brake fluid contaminant.

In addition, air contains water vapor. As mentioned earlier, water can degrade brake fluid. Water also contains oxygen, which contributes to its effects on the brake system.

Solid Particles

These include dirt and dust. Depending on their size and hardness, they can damage brake parts, increasing the risk of corrosion and failure. Once they accumulate in considerable amounts, they can also interfere with brake fluid flow, reducing the braking performance.


Rust can form on steel parts exposed to oxygen. Flakes of rust can separate from the corroded part and spread throughout the brake system.

How Can Contaminants Enter the Brake System?

Your car’s braking system is a closed system that keeps contaminants out. Nothing is perfect, however. There are ways for air, water, and other contaminants to enter the system and contaminate the brake fluid, especially in older vehicles that have seen heavy use.

Unless your brake system has been compromised by leaving the master cylinder cap off or pouring the wrong kind of fluid (like oil or power steering fluid) in the cylinder reservoir, brake fluid contaminants are typically going to be copper and water.

To learn how to test if your brake fluid needs flushing, read our discussion about brake fluid contaminants.

Heavy Braking

Do you often use the brake system? Or do you hit the brakes hard? Driving patterns that require constant and heavy use of the brakes can increase the risk of contaminated brake fluid.

The braking process generates friction to decelerate your vehicle’s wheels. In turn, friction produces heat. As your car moves faster, it requires more friction to slow down or stop, leading to hotter temperatures in the brake system.

Unfortunately, heat causes moisture to condense within the brake system. The higher the temperature, the heavier the condensation. Thanks to condensation, water can enter the brakes and degrade brake fluid.

Heavy braking also puts more strain on parts like brake lines and seals. These brake parts work harder, so they wear out faster. A worn-out part can leak brake fluid and provide openings for contaminants to enter the brake system.

Leaks in the Brake System

Contaminants can also enter through leaks in damaged or loosened parts, such as brake lines and seals. Miniscule leaks not only allow brake fluid to escape, but they also allow air and water to enter the brake system. Larger holes can admit larger solid particles and more contaminants.

During Brake System Maintenance

It’s important to keep your car in good condition. However, repairing brake problems or replacing bad brake parts can lead to dirty brake fluid that you must deal with.

Whenever you work on the brake system, you open a hole in its defenses. Topping off the brake fluid reservoir requires you to remove its cap, which can allow air and moisture in brake fluid.

The same applies for replacing bad parts like the brake shoes. Removing a part exposes its joints and seals, allowing contaminants to enter the temporary breaches in the brake system.

What Are the Common Symptoms of Contaminated Brake Fluid?

Since brake fluid is an intrinsic part of the brake system, it will cause noticeable effects when contaminated by substances like air, water, and solid particles.

Here are the most common symptoms of contaminated brake fluid:

How Can You Avoid Dirty Brake Fluid?

Contaminated brake fluid is bad news for your vehicle. Fortunately, there are ways to block contaminants from entering the brake system and ruining the hydraulic fluid.

Avoid hard braking unless you have no other choice. Keeping the brakes cool reduces the chances of condensation inside the brake system.

Always inspect your car’s bottom and the ground beneath it for any signs of leaking brake fluid. Once you find a leak, don’t delay to patch the damage or replace the leaky part.

After replacing a bad brake part or running maintenance on the brake system, flush the brakes to get rid of accumulated contaminants.

What Should You Do to Fix Brake Fluid Contamination?

In most cases, you must flush your car’s brake fluid to eliminate contaminants. Brake flushing removes the old hydraulic fluid and replaces it with a fresh batch. You can perform it yourself or have a certified mechanic perform the job.

After a brake flush, you might also need to bleed your brakes. Brake bleeding gets rid of air bubbles that can reduce braking performance.

Learn more about brake flushing by reading our article on the process.

How to Get Quality Replacement Brake Fluid for Your Vehicle

Brake fluid is an integral part of your vehicle’s brake system. If your brake fluid has gone bad, then it’s very important for you to flush and replace it as soon as possible. Having bad brake fluid in your brake system can easily lead to bad accidents where the brakes don’t engage when they should. Thankfully, you can purchase quality brake fluid at aims to make ordering new brake fluid and other parts as easy and convenient as possible. That’s why our easy-to-navigate website is designed so you can order what you need in a few easy clicks. At the same time, we want you to have confidence when ordering online. Our price match guarantee helps ensure that you’re getting the best deals on our site. If you have any questions or concerns, we’re happy to help around the clock. Simply chat with our friendly and helpful customer service team.

Place your order for a new bottle of brake fluid on today

About The Authors
Written By Automotive and Tech Writers

The Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Reviewed By Technical Reviewer at

Richard McCuistian has worked for nearly 50 years in the automotive field as a professional technician, an instructor, and a freelance automotive writer for Motor Age, ACtion magazine, Power Stroke Registry, and others. Richard is ASE certified for more than 30 years in 10 categories, including L1 Advanced Engine Performance and Light Vehicle Diesel.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

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