Let’s Talk About Your Brake Fluid, Because No One Else Is

Sitting in the dark corner of neglected, recommended service intervals and tucked away between your cabin air filter and your differential fluid sits the lowly and ever humble workhorse; your brake fluid. We wouldn’t be surprised if 75-percent of the vehicles on the road are still using the factory-installed fluid — and the ones who aren’t, probably only changed it because their mechanic sternly suggested it (billable hours, baby!).

Brake Fade

Many people are surprised to hear this, but you can easily boil your brake fluid. Why is that bad? Boiled fluid results in creating air bubbles in the brake lines. While brake fluid itself is not compressible, as we all know from forced induction, air is very compressible. 

Those bubbles result in a squishy brake pedal and a reduced squeezing force on the rotors. It can get so bad, you can push the pedal to the floor and still not be able to stop your car. That’s not what you want mid-corner at Laguna Seca race track, and that certainly isn’t what you want coming down a mountain pass with your family in the car.

To get into how this happens, we need to go over the substances that comprise brake fluid, and the role it plays in actually stopping you.

Pascal’s law Of Fluid Dynamics (bear with me, we’re going to get nerdy):

Your car’s brakes — just like any other hydraulic system — take advantage of Pascal’s law as its means of force amplification. Pascal’s law basically states: any pressure change enacted on an incompressible fluid will be transmitted equally throughout the fluid in every direction. That makes sense, right? But how do we use this principle to our advantage?

brake fluid

Image credit: Wikipedia

In the diagram above, we have a U-shaped tube with a piston on each side. The tube is airtight and filled with water (an incompressible fluid). Any pressure applied to the one side will be transmitted through the fluid to push the piston on the other side. If we make the surface area of the right piston 10 times the surface area of the left piston, then 10 times the pressure will be transmitted to the right piston. Ten pounds of pressure on the left piston results in 100 pounds of pressure on the left.

In our cars, we apply that pressure via the brake pedal as it pushes against the brake booster — which contains a large surface-area piston — through the master cylinder and brake lines to the smaller surface area pistons in the calipers. 

First Rule Of Brake Fluid: Water Is Bad

brake fluid

I couldn’t resist…

If water is incompressible, can it be used as brake fluid? Yes, it absolutely can! But, not for very long. Incompressibility is just one of brake fluid’s four jobs: 

1: Be incompressible

2: Resist high temperatures without boiling

3: Act as a lubricant

4: Be anti-corrosive

Water only achieves the first requirement and is not very good at the other three. For that reason,  brake fluid consists of a special formulation of glycol-ether to accomplish all four of those things. Water will, however, make its way into your brake system, and this is “no bueno.” 

Brake fluid is highly hygroscopic, which means it absorbs water. This absorption can occur through the rubber brake lines, various seals around the system, or if you open the reservoir to add fluid. This happens less than it used to with modern brake lines, but it still happens. That’s why fluid manufacturers list two boiling temperatures for their fluids. A dry boiling point and a wet boiling point:

Dot 3: 

Dry boiling point: 401 degrees 

Wet boiling point: 284 degrees

Dot 4:

Dry boiling point: 446 degrees 

Wet boiling point: 311 degrees

Dot 5 and 5.1 (more on these later):

Dry boiling point: 500 degrees

Wet boiling point: 356 degrees

It seems strange to call a fluid wet or dry, but basically, dry fluid is fresh stuff right from the container, and wet is fluid that has been contaminated with 3.7-percent water. According to Amsoil, DOT 3 fluid can absorb up to two-percent water content in a year. You read that correctly. With just 3.7-percent total water content, your high-performance DOT 4 or 5.1 fluid turned into something worse than the DOT 3 that comes in your neighbor’s Prius. 

Once you boil your fluid, you can pump your brakes to regain some pedal feel in the moment, but your brakes will need to be bled to get that air out, and it’s likely a sign that it’s time to change out your fluid completely.

When To Change Your Brake Fluid:

Temperature resistance isn’t the only reason to change your fluid. Too much water in the system can lead to corrosion of metal components, like iron calipers. That build-up of corrosion leads to interference with moving parts — possibly to the point of a seized caliper. Even worse, the included anti-corrosive compounds in the fluid break down over time, compounding the issue.

That’s why shops will offer to test your brake fluid for water and copper content. Copper seems like a strange thing to make its way into your brake lines, but copper content in your brakes is an indicator of corrosion. If you’ve done enough work on your cars, you’ll know that copper crush washers are used in fittings in the brake system, and those are some of the first components to corrode. 

The Automotive Maintenance and Repair Association (AMRA) recommends changing your brake fluid when its copper content reaches 200 parts per million (PPM). We would take that number with a grain of salt, though, because that organization is made up of companies who have a financial interest in you flushing your brake fluid often. Either way, save that little fact in your head for some obscure Jeopardy question someday. 

What Brake Fluid To Use?

brake fluid

Yes, I keep brake fluid on my desk. No, I don’t know why.

That depends on what you’re doing with your car. First and foremost, check what your manufacturer recommends, and don’t use anything less than that rating. If your car is just a commuter and you don’t do much in the way of performance driving or towing, then you’ll be fine with DOT 3. If you like doing the occasional canyon run, track day, or tow a trailer fairly often, you should probably use DOT 4. 

If you do any heavy towing, regularly race your car on brake-intensive tracks, or are a character in the Fast and Furious franchise, then maybe DOT 5.1 would be better for you.

Are DOT 3, 4, and 5.1 all made with a similar glycol-ether — just with different temperature ratings — and are all okay to mix in a pinch? No, DOT 5 is a completely different beast. It’s a silicone-based fluid that is not hygroscopic and can be distinguished from other brake fluid by its purple color. It’s dyed that way to avoid mixing the fluid. In other words, DOT 5 is not compatible with other fluids listed here. It’s used mostly in applications where a car might sit for a while. It does not play nicely with ABS systems and is quite expensive. It’s not something most people really want to consider, despite the higher temperature rating.

brake fluid

Image credit: Hagerty.com

Now you’re armed with yet another barrage of semi-useful, but wholly uninteresting facts to regurgitate at the next social gathering of your choice. You are now more of an expert on brake fluid than 99.99-percent of people on this planet. Doesn’t it feel good? We think it feels good.

Is there anything I missed? Do you disagree with, or have any other topics you’d like to see us cover? Leave a comment below or drop me a line at [email protected].

About the author

Garrett Davis

Garrett has something of a sickness when it comes to cars, working on everything from Jeeps, to sports cars, to over-engineered German nightmares. Currently he is embroiled in an Audi Allroad offroad project, and is slowly losing his grasp on sanity.
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