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The basics of lubricants: the ins and outs of brake fluid (7/9)

Brake fluid is yet another fluid that your vehicle could not do without for a minute! Read on to understand why it's so important, and what to remember when choosing and/or changing brake fluid.

Technical Expertise

We're happy to show you around the world of automotive fluids in our 'Basics of lubricants' series. Today, we're zooming in on brake fluid. Read on and become an instant expert!

Why do you need brake fluid anyway?

Your brakes keep you alive every day out on the road, and brake fluid is vital to the performance of both your braking and clutch systems. This is true whether you drive an on- or off-road vehicle, a car or a van, a motorcycle or a truck.

Why it's important to keep a good eye on the state of your brake fluid

Like any fluid in your vehicle, your brake fluid slowly becomes contaminated over time:
  • Moisture and air contamination can creep in through weak seals or during routine maintenance.
  • The fluid in your caliper or wheel cylinder may cook off from the high temperatures of braking.
  • Hoses or lines may erode, adding foreign material to the fluid.

A reduction in responsiveness or even brake failure are the less desirable outcomes of this contamination. 

One of the most common contaminants of braking fluid are 'air pockets'. During maintenance a mechanic should 'bleed' your brakes, meaning that he flushes out all air pockets trapped in the brake system. If not done properly, brake fluid can suffer from 'air contamination'. Aging seals or other leaking components enable air to sneak in, leaving you with a spongy feeling when pressing down on the brake pedal. 

Contamination with moisture also undermines your brakes’ performance. Brake fluids are designed to absorb moisture, preventing loose water in the system from eroding components. But the more moisture your brake fluid absorbs, the lower its boiling point becomes. As a result the brake fluid van evaporate and form air pockets, leading again to a spongy brake pedal.

What do the classifications of brake fluid mean?

The U.S. Department of Transportation (DOT) sorts brake fluids into a series of classifications: DOT ¾, DOT 5 and DOT 5.1 are the most used categories these days. DOT ¾ and DOT 5.1 are 'hygroscopic,' meaning that they absorb airborne moisture. DOT 5 has a silicone base and is not hygroscopic.

Brake fluids are classified first and foremost by their boiling points. Because the brake pads create enormous friction when they press against the drum or disk, your brakes experience very high temperatures. If the fluid does hit its boiling point, it evaporates and becomes compressible, meaning that your hydraulics will stop functioning properly.

The DOT measures both “dry” boiling point (uncontaminated fluid straight from the container) and “wet” boiling point (the fluid after it’s absorbed some water).

The next important property is the brake fluid’s viscosity (or thickness). All brake fluids must meet SAE specifications at minus 40 degrees C and 100 degrees C. Beyond this requirement, minimum and maximum viscosities vary between the classifications.

How to choose a brake fluid for purchase

First, you’ll need to choose what classification of brake fluid to buy. Don’t waste a lot of time comparing specs—the best guide is just to stick with whatever’s already in your system. It depends on your vehicle, but a higher classification of brake fluid won’t necessarily give you better performance.

After you’ve made your selection, determine how much you need. Don’t fall for the false economy of buying a larger bottle, planning to use some of it later. In reality, this remainder will end up contaminated by airborne moisture, and you won’t want to use it for top-offs.

Get rid of these contaminated leftovers—or better yet, don’t buy more than you need in the first place. To make this more convenient, Wolf offers brake fluid in a variety of sizes, including 250 ml, over 500 ml, and 1 L.

Other concerns when buying brake fluid

When buying, you’ll want to make sure the package is airtight, with its foil intact. If the seals are broken, this means that the fluid has already been exposed to air and moisture, reducing its effectiveness and lifespan.

Finally, keep in mind that you should never “mix and match” classifications of brake fluid. All fluids that fulfil DOT requirements are compatible, but mixing a DOT 3/4 fluid with a DOT5.1 fluid will give your fluid a new, unknown chemical composition. This mixture may no longer be compatible with all your system’s components. If you decide to switch fluid classifications, be sure to cleanse your system before adding the new fluid.

Wolf offers top-quality brake fluids for every need—click through to our website for details!


  • Brake fluid is vital to the performance of both your braking and clutch systems.
  • Like any fluid in your vehicle, your brake fluid slowly becomes contaminated over time and needs regular maintenance.
  • The U.S. Department of Transportation regulates brake fluid classifications by boiling point, viscosity, and other properties.
  • To determine what classification to buy, look at what’s already in your system.
  • Don’t fall for the false economy of buying a larger bottle, you won’t want to use leftovers for top-offs.
  • Never mix different classifications.

Missed our last article of this series? Here’s a shortcut !

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