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While brake fluid is one of the most important components in your vehicle, it doesn’t seem to get much attention from the average consumer. After all, many vehicles can go over 100k miles without any recommended brake fluid service, and coupled with modern brake pads (which are as about as advanced as any system on your vehicle), vehicle braking systems are reliable and relatively worry-free.

Brake fluid is important

Most people think the engine is the most important part on a vehicle, but any racer will tell you it's the brakes that count.

Perhaps as a consequence of this great reliability, vehicle owners who need to purchase brake fluid often aren’t familiar with what the DOT 3, 4, 5, and 5.1 ratings found on fluid actually mean.

There are a number of technical factors that go into the ratings (most of which don’t need to be discussed), there are two factors used to set a fluid’s final DOT rating that consumers should be aware of:

  1. Moisture resistance
  2. Heat resistance

A brake fluid’s ability (or inability) to resist damage from heat and moisture has a large impact on the performance of the fluid, and on the performance of your vehicle’s braking system.

How Brake Fluid Can Boil

Even though your brake fluid is heavily resistant to boiling over (both because of the chemical composition of the fluid and the pressure inside your vehicle’s brake system), there are situations where brake fluid can boil. If/when this happens, you’ll find your car doesn’t stop nearly as quickly as it should. In fact, a vehicle with boiled brake fluid can feel like the brakes have failed, as maximum pedal pressure will have minimal impact on speed.

Brake Fluid DOT Ratings

Higher DOT ratings aren't always better - you need the rating that your vehicle manufacturer recommends.

Boil-over occurs when the brake fluid gets so hot that it exceeds it’s boiling point. For most drivers, boil-over will likely occur

  1. During heavy towing or hauling
  2. During long descents or while driving in mountainous areas
  3. During very aggressive driving (like racing)

When you use your brakes heavily the fluid inside the system is compressed and decompressed over and over, and this causes friction inside the fluid. Friction leads to heat, and at some point – be it 400°F or 500°F – your brake fluid will boil.

DOT brake fluid ratings are a basic indication of a brake fluid’s resistance to boil-over. DOT 3 has a boiling point of 401°F, DOT 4 has a boiling point of 446°F, while DOT 5 and 5.1 are resistant to boil-over up to 500°F. For most vehicles and most situations, DOT 3 offers plenty of heat resistance.

However, there’s more to brake fluid than heat resistance.

Moisture Can Ruin Brake Fluid

Because of its chemical composition, brake fluid absorbs moisture very easily. If brake fluid is exposed to too much moisture, it will become almost useless. The reason? As moisture is added to brake fluid, the boiling point drops. A little bit of moisture can take a 400°F boiling point down to 250°F, a temperature that’s so low it’s a safety risk.

Unfortunately, “too much” moisture isn’t actually very much. Brake fluid is hydrophilic, which means it is attracted to water. If you were to leave the cap off your brake fluid reservoir for an extended period, it would literally soak up moisture in the surrounding air. Depending on the humidity in the air, the boiling point could change with just a few minutes of exposure to the atmosphere. Even having just four percent water saturation in your brake fluid can severely hinder performance…which brings us back to the DOT rating.

A common misconception is that DOT ratings correspond directly to the quality of a brake fluid. While there is some truth to that notion, this doesn’t mean that consumers can choose to downgrade brake fluid whenever they like.

  • All brake fluids are backwards compatible by law – they’re required to be able to meet or exceed the requirements of the next lowest level.
  • If your car’s braking system has a DOT 4 rating (which is typical of newer vehicles), it can only accept brake fluid rated DOT 4 or higher.
  • Even though brake fluids are backwards compatible, it’s always best to you fill your brake system with the DOT-rated fluid recommended by the vehicle manufacturer. This ensures consistent performance.

When the time comes to change your vehicle’s brake fluid, consult your owner’s manual, follow the recommended service procedure, and remember that brake fluid loves moisture…and that moisture ruins brake fluid.


About the author: Jason Lancaster


Jason is the editor and founder of,, and a handful of other automotive enthusiast websites. When Jason isn't writing about cars, he's playing or watching basketball, hanging out with his family, and finding new ways to waste his hard-earned dollars on his car.


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