Does Brake Fluid Go Bad?

Brake fluid absorbs moisture and accumulates contaminants, meaning it eventually goes bad and needs changing.

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AMSOIL Synthetic Brake Fluid
by John Baker
September 18, 2017

Here is the short answer: Yes, brake fluid can go bad.

Brake fluid absorbs moisture, which reduces its performance. For that reason, it’s best practice to change brake fluid every two years. However, brake fluid is one of the maintenance items that drivers miss most often.

People rarely change their brake fluid before it goes bad. They often wait until they hear the grinding sound from the wear indicator on the brakes, change the brake pads and bleed the brakes in the process.

Why Does Brake Fluid Go Bad?

Brake fluid is hygroscopic. That means it likes water, just like a sponge. In fact, brake fluid is notorious for absorbing water.

Common brake-fluid testing devices on the market solely test for moisture in the fluid. Here comes the kicker: even if you do everything in your power to prevent moisture absorption, you can still have issues. Brake fluid can absorb moisture in a variety of ways – through the packaging process, while pouring it into the reservoir and even through the brake fluid lines.

How Bad is “Bad”?

Since brake fluid is hygroscopic, required testing simulates real-world field conditions.

One of the key testing parameters is the brake fluid’s boiling point. The wet & dry boiling points are tested to help communicate the quality of a brake fluid. Check out this post for more, but here is a quick description of boiling points:

Dry boiling point (ERBP): Measures the boiling point of the fresh fluid right out of the bottle before it has been exposed to, and contaminated by, moisture.

Wet Boiling point (WERBP): Measures the boiling point of fluid after it has had time to absorb moisture from its surroundings, similar to conditions under which the average vehicle operates (3.7% water contamination, to be exact).

AMSOIL DOT 3 & DOT 4 Synthetic Brake Fluid is purpose-built to perform in wet conditions. With a wet boiling point of 368°F (187°C), it far exceeds the minimum requirements of 284°F (140°C) for DOT 3 fluid and 311°F (155°C) for DOT 4 fluid.

How Long Does Brake Fluid Last?

In ideal conditions, an unopened bottle of brake fluid lasts about two years. It’s best to use a new bottle of brake fluid every time you need it because the fluid attracts moisture as soon as it is opened.

Once in your vehicle, brake fluid life is based on the application in which it’s used. Operating conditions – and, thus, fluid life – vary depending on the environment, equipment type and application.

In high-humidity areas, moisture is absorbed through the hoses and seals. Racers change brake fluid more often due to significantly higher operating temperatures. To that point, the dry boiling point tends to matter more. That’s why we formulated AMSOIL DOMINATOR® DOT 4 Racing Brake Fluid to deliver an impressive 580°F (304°C) dry boiling point.

AMSOIL Performance Testing

Laboratory tested and proven on the road.

See how AMSOIL products perform in the lab and in the field.

Check out the tests

Has Your Brake Fluid Gone Bad?

Although brake fluid absorbs water like a sponge, you don’t want your pedal to feel like a sponge when you jam on the brakes.

A vehicle using fresh brake fluid should deliver solid brake-pedal feel, like you’re pressing down on a brick. Think of the last time you drove a new car. The brakes likely inspired confidence and felt rock-solid. That’s because the entire system was brand new and moisture had yet to infiltrate the fluid.

There are only a couple ways to ensure that brake fluid lasts.

First, change it at least every two years, or when you change brake pads, and be mindful of the amount of time you expose the product to the environment.

Another is by using AMSOIL DOT 3 & DOT 4 Synthetic Brake Fluid. It is packaged with nitrogen, which prevents moisture contamination natural to the packaging process.

Additionally, it is engineered with high boiling points to exceed the minimum standards, which translates into solid, confident braking next time that deer jumps in front of you or the guy texting ahead of you suddenly stops.

If it’s time to bleed your brakes and change the fluid, find out how to do it here.

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by John Baker

AMSOIL Technical Writer and avid avid DIYer with 12 years in the synthetic lubricants industry, who enjoys making technical topics in the automotive, powersports and industrial markets easy to understand.

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AMSOIL Synthetic Brake Fluid