Brake tech is a very complex and nuanced topic, so we asked the experts at Alcon Brakes to help us clarify and expand on some frequently asked questions in the braking universe.
Today, they’re taking a deep dive into brake fluid by comparing the boiling points between common DOT brake fluid types and explaining some other important brake fluid considerations to help you choose the best brake fluid for your specific vehicle application.
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When your brake fluid boils, it significantly compromises your vehicle’s ability to make a safe, controlled stop.
So…why don’t just make brake fluid that boils at a higher temperature?
The solution isn’t that simple.
That’s because a brake fluid’s boiling point is only one of many factors—for starters, seals and boots within your brake system have maximum allowable temperature thresholds too. More importantly, running hotter fluid in a brake system can seriously shorten the lifespan of some caliper housing materials.
In other words, you cannot address fluid boiling points in isolation.
As a result, brake system engineers developed a few common brake fluid types. In the United States, you’ll typically see DOT 3, DOT 4, DOT 5, and DOT 5.1.
Vehicle engineers will pick the most suitable brake fluid type for the application and you’ll be able to find that spec’d out in the owners manual and, often times, printed onto the master cylinder reservoir itself.
Brake Fluid Boiling Points
| Fluid Type | Boiling Point (Dry) | Boiling Point (Wet) | Chemical Composition |
|---|---|---|---|
| DOT 3 | 205° C / 401° F | 140° C / 284° F | Glycol Ether |
| DOT 4 | 230° C / 446° F | 155° C / 311° F | Glycol Ether/Borate Ester |
| DOT 5 | 260° C / 500° F | 180° C / 356° F | Silicone |
| DOT 5.1 | 260° C / 500° F | 180° C / 356° F | Borate Ester/Glycol Ether |
It’s Not Just About Boiling Points
The formulation of the various types of brake fluids is extremely technical. They have a number of properties that allow for safe operation.
Low temperatures need to be considered as well as high. The fluid needs to maintain constant viscosity throughout a wide temperature range (including sub-zero temperatures).
Temperature aside, compressibility needs to be minimized. And your brake fluid needs to be compatible with all other components in the brake system—which is particularly important if you’ve got an older car with aftermarket or otherwise modified brake components.
Using a brake fluid type other than what the manufacturer specifies could lead to brake failure due to incompatibility with brake seals and stability control systems. It could also lead to corrosion and accelerated wear of brake components.
What’s the Difference Between Wet & Dry Boling Points (& Why It Matters)
When looking at boiling points, you’ll discover both wet and dry specs. That’s because most brake fluids are hydroscopic—which is a fancy way of saying it absorbs water over time. And, as the brake fluid absorbs water, its boiling point is reduced.
The dry boiling point is when the fluid is new.
The wet boiling point is defined as containing 3.7% of water by volume. In the case of DOT 4 fluid, for example, the boiling point will reduce from 446 Degrees F (dry) to 311 degrees F (wet).
How to Choose the Best Brake Fluid
The key message of this post is to only use the standard of brake fluid dictated by the vehicle manufacturer or brake component supplier and to replace the fluid at the recommended intervals.
Brake fluid should be replaced during any brake service or around the two-year mark to maintain boiling
Dave Matthews – Summit Racing Technical Advisor
point and proper compressibility, and combat corrosion.
Brake engineers will have selected the best fluid for the application and carried out extensive testing to ensure that it is safe in all reasonable circumstances.
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