One of the best ways to ensure the health of your brake components is to get your brake fluid changed regularly. While you may not feel a difference after a brake fluid flush, it will help you maximize the service life of your brake parts and save you from costly repairs.
Brake fluid is a hydraulic agent formulated to protect your vehicle’s metal brake components from moisture and corrosion. It is a polyglycol (polyalkylene-glycol-ether) mixture that is clear to amber in color. The main purpose of this fluid is to transfer the energy from the depressed pedal to the brake calipers.
The Society of Automotive Engineers (SAE) and the Department of Transportation (DOT) have established specification standards for automotive brake fluid. They must have a high boiling point, a low freezing point, and they must not damage rubber parts within the brake system. All products must meet the specifications listed in Federal Motor Vehicle Safety Standard (FMVSS) 116.
Nearly all cars use brake fluid to activate the brakes. Because the braking system uses a liquid to create force, it’s known as a hydraulic system.
When you apply the brake pedal, a device called the master cylinder transfers force from your foot to the brake fluid. In vehicles that use the ABS hydraulic control unit as a master cylinder, it’s the hydraulic ABS pump that transfers the force.
From there, the pressurized fluid flows to a brake caliper (or wheel cylinder in a drum brake system) at each wheel. The fluid forces the caliper to squeeze the brake pads against the rotor or push the wheel cylinders against the brake shoes and into the drum. This creates the friction needed to slow your wheels down and bring them to a stop.
Brake fluid needs to have a high boiling point in order to avoid vaporization and has to be incompressible for the system to properly work. When the level of brake fluid is low, depressing the brake pedal may not activate the brake caliper or wheel cylinder.
The brake fluid is hygroscopic, which means it can absorb moisture. The US Department of Transportation (DOT) issues the standards for all motorized vehicle brake fluids in the country and rates them by their base component.
There are four types of hydraulic brake fluid: DOT 3, DOT 4, DOT 5, and DOT 5.1.
DOT 3 brake fluid is glycol-based and is the most common type of brake fluid. It is relatively cheap due to its low boiling point.
This type of brake fluid is also glycol-based. It has a higher boiling point than DOT 3 brake fluid and has moisture resistance properties.
DOT 5 brake fluid is silicone-based and works well at resisting moisture.
One thing to note about this type is that it can’t be mixed with the glycol-based DOT 3 and DOT 4.
DOT 5.1 is a non-silicone-based polyglycol fluid. Like DOT 5, it works well at resisting moisture.
It is made with a polyglycol base that gives it a dry boiling point that’s over 500ºF.
This brake fluid can be mixed with DOT 3 and DOT 4 fluid (when the vehicle manufacturer specifies that doing so is safe).
Brake fluid has two boiling point ratings: dry and wet. The dry boiling point pertains to the boiling point of fresh brake fluid in an unopened container. The wet boiling point, on the other hand, refers to the fluid’s boiling point after it has accumulated water, which begins at the start of its service life.
| Brake Fluid Type | Dry Boiling Point | Wet Boiling Point |
| DOT 3 | 401 degrees Fahrenheit | 284 degrees Fahrenheit |
| DOT 4 | 446 degrees Fahrenheit | 311 degrees Fahrenheit |
| DOT 5 | 500 degrees Fahrenheit | 356 degrees Fahrenheit |
| DOT 5.1 | 500-600 degrees Fahrenheit | 356+ degrees Fahrenheit |
Most vehicle manufacturers recommend changing the brake fluid as part of routine maintenance. On average, it is changed every two years or every 30,000 miles, whichever comes first.
The owner’s manual has all the information you need–from the type of brake fluid compatible with your vehicle to the manufacturer’s recommended service interval. By changing your brake fluid on schedule, you can prevent premature wear and corrosion damage caused by contaminated brake fluid.
Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.