Brake fluid is a liquid that transmits brake-pedal force directly to the brake caliper pistons. As brake fluid is not compressible, force applied to the fluid in one location (master cylinder) is immediately transferred throughout the fluid system equally.
The magic of hydraulics means brake pedal force applied at the master cylinder may be multiplied at the wheel by enlarging the brake caliper piston, and that’s exactly how car brakes work.
But none of this would be possible without brake fluid; it’s the system’s lifeblood.
Where’s Brake Fluid Located?
Brake Fluid Lines and Reservoir
Brake fluid is located throughout the hydraulic brake system, which stretches along the chassis from front to rear wheels and can be viewed in the brake fluid reservoir, which is positioned under the hood directly behind the brake pedal assembly on the firewall.
What Does Brake Fluid Do?
Brake fluid is tasked with three main jobs:
- Transmit brake pedal force – The point of brake fluid is to transmit brake pedal force quickly to the brake calipers, where they can clamp the brake pads to rotors and slow your vehicle.
- Absorb moisture – Brake fluid is hygroscopic, which means it attracts and absorbed moisture. Absorbing moisture isn’t a bad trait; all brake systems contain some moisture, and a fluid that can handle it is a bonus.
- Resist heat – Brakes are all about heat, and one of the fluid’s top talents is heat management; in fact, it is so important brake fluid is categorized by its ability to resist heat.
Brake fluid has other tasks, too, like lubricating the brake system seals, helping to keep them supple, and reducing wear & tear and anti-corrosive additives.
8 Things to Know About Brake Fluid
- Brake fluid should be maintained between fluid reservoir Min and Max marks.
- A low brake fluid level usually means the brake pads are worn and need replacing.
- Brake fluid absorbs about 1% moisture yearly, and brake performance suffers when moisture content exceeds 3% total.
- Brake fluid needs to be changed every 2 to 3 years.
- There are two types of brake fluid, Glycol based (which is DOT 3, 4, 5.1) and Silicone based (which is DOT5). Do not mix Glycol based with Silicone based brake fluid.
- Ideally, Glycol (DOT 3, 4, 5.1) shouldn’t be intermixed, but you may if you’re in a pinch.
- The brake fluid type is specified on the brake fluid reservoir cap.
- Brake fluid moisture may be tested in three ways, refractor meter, dip strips, or electronically.
Brake Fluid Problems
The two main enemies of brake fluid are heat and moisture. Heat is a by-product of slowing a vehicle; it’s just a fact vehicle brake systems have to deal with.
The second brake fluid enemy is moisture. Glycol-based fluids attract moisture; the problem arises when the moisture content exceeds 3%; the heat causes the moisture inside the brake fluid to boil and become steam, and steam, unlike brake fluid, is compressible, meaning the brake pedal sinks to the floor and the brakes are dangerously compromised.
The number one Brake fluid problem – Moisture contaminant brake fluid (Old fluid).
Brake fluid should be tested yearly and changed every two years three max.
Brake Fluid Types
All brake fluid is marked with the standardized DOT (Department Of Transport) specification, and all vehicle brake fluid reservoir caps are marked with your vehicle’s recommended DOT specification.
It’s important to use the type specified on the cap; there are, as said earlier, two families of brake fluid, and they don’t like to be mixed; they are Glycol based and Silicone based.
| Fluid Type | Compounds |
| DOT3 | Glycol – Ether |
| DOT4 | Glycol – Ether Borate – Ester |
| DOT5.1 | Glycol – Ether Borate – Ester |
| DOT5 | Silicone |
The DOT category relates to the boiling point of the fluid. The higher the DOT number, the higher the boiling point. The boiling point is, as you know, important, brake pads and rotors make a ton of heat, and that heat can cause brake fade in vehicles with moisture-laden brake fluid.
Preventing the fluid from boiling is the first defense, so fluid is categorized by its ability to withstand heat; it’s tested both dry (new fluid moisture-free) and wet (old fluid with 3.7% moisture).
Brake Fluid Compatibility Chart
Next, we’ll look at the two families of brake fluids, Glycol and Silicone.
Glycol Based Brake Fluid
Glycol-based brake fluid is most commonly found in passenger vehicles on the road. Three different Glycol specs offer different boiling point performances. Ideally, they shouldn’t be mixed, but they can be when you’re in a pinch.
Glycol brake fluid attracts moisture (hygroscopic) at a rate of 1% per year, so the fluid needs to be changed, and the brake system flushed every two to three years max. Brake fluid is important as moisture in old brake fluid turns to steam as the brakes heat up and that, in turn, results in brake fade.
| Glycol Fluid Category | Dry Boiling Point F° | Wet Boiling Point F° |
| DOT3 | 401 | 284 |
| DOT4 | 446 | 311 |
| DOT5.1 | 500 | 356 |
Silicone Based Brake Fluid
DOT5 Silicone-based fluid is a maintenance-free fluid, meaning unlike Glycol-based fluid, it doesn’t attract moisture and doesn’t need changing. All military vehicles use Silicone based brake fluid.
Silicone is slightly more compressible than glycol-based fluid, but the lack of maintenance more than makes up for this.
| Silicone Fluid Category | Dry Boiling Point F° | Wet Boiling Point F° |
| DOT5 | 500 | 356 |
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John Cunningham is an Automotive Technician and writer on Rustyautos.com. He’s been a mechanic for over twenty-five years and has worked for GM, Volvo, Volkswagen, Land Rover, and Jaguar dealerships.
John uses his know-how and experience to write articles that help fellow gearheads with all aspects of vehicle ownership, including maintenance, repair, and troubleshooting.