Types of Brake Fluid in Cars
Brake fluid is a hydraulic fluid used in vehicle brake systems to transfer force into hydraulic pressure. When the brake pedal is pressed, the master cylinder pressurizes the brake fluid, which flows through the brake lines to the wheel cylinders and calipers to apply the brakes. Proper brake fluid is crucial for safe braking and brake system performance.
There are several different types of brake fluid that meet various specifications and quality standards. The most common types used today are DOT 3, DOT 4, DOT 5.1, and DOT 5 brake fluids. Each has different properties in terms of compressibility, viscosity, wet and dry boiling points, and chemical composition. Using the manufacturer recommended brake fluid type for a vehicle’s brake system is important to avoid any issues with brake failure or degradation of components over time.
This article will provide an overview of the major types of brake fluid used in cars, key differences between them, when fluid should be changed, and proper handling and storage of brake fluid. Proper selection and maintenance of brake fluid is a key part of keeping a vehicle’s brake system performing safely.
DOT 3 Brake Fluid
DOT 3 brake fluid is the most common type of brake fluid used in vehicles today. It meets the minimum standards required by the U.S. Department of Transportation for use in hydraulic brake systems as outlined in Federal Motor Vehicle Safety Standard (FMVSS) No. 116 (per the Castrol Brake Fluid DOT 3 specifications).
Key characteristics of DOT 3 brake fluid include:
- Minimum wet boiling point of 401°F (205°C)
- Compatible for use with anti-lock braking systems (ABS)
- Made from polyethylene glycol ether-based formula
DOT 3 has a relatively low dry boiling point, but meets the basic requirements for most passenger vehicles and light trucks that don’t demand heavy-duty braking performance (per the Sinclair DOT 3 specifications). It provides adequate protection against vapor lock, caliper corrosion, and brake fade.
DOT 4 Brake Fluid
DOT 4 brake fluid has a higher boiling point than DOT 3, typically with a minimum wet boiling point of 279°F and dry boiling point of 446°F compared to DOT 3 which has minimum wet and dry boiling points of 205°F and 401°F respectively (Source). The higher boiling point makes DOT 4 better suited for performance driving applications where brake temperatures can get quite high during repeated heavy braking.
The glycol ethers in DOT 4 also provide more resistance to brake fade, which is a reduction in stopping power after repeated brake use (Source). This fade resistance allows the brakes to maintain stopping power even after continuous heavy braking, like when driving on a race track. For these reasons, DOT 4 fluid is recommended for track days, autocross racing, and other competitive driving where brake temperatures will be high.
DOT 5 Brake Fluid
DOT 5 brake fluid is made from a base of silicones and polyglycols for a higher boiling point than other types of brake fluid. Silicones are saturated synthetic compounds that contain silicon-oxygen bonds. The key property of silicone-based brake fluid is its much higher boiling point of over 500°F compared to DOT 3 and DOT 4 fluids which boil around 401°F and 446°F respectively. This exceptionally high boiling point prevents vapor lock, making DOT 5 brake fluid well-suited for performance applications where brakes can overheat during hard driving [1].
However, DOT 5 fluid is not compatible with some brake components found in many vehicles. The silicone can cause swelling of rubber seals and lead to leaks over time. For this reason, DOT 5 fluid should only be used in brake systems specifically designed for its use. Most consumer vehicles are designed for DOT 3 or DOT 4 brake fluid. Always check manufacturer specifications before using a different type of brake fluid [2].
DOT 5.1 Brake Fluid
DOT 5.1 brake fluid is glycol ether based like DOT 3 and DOT 4 brake fluids. It has a higher minimum wet boiling point of 280°C (536°F) compared to DOT 4 which has a minimum wet boiling point of 230°C (446°F) [1]. This higher boiling point allows DOT 5.1 to withstand extreme braking conditions better without vaporizing.
DOT 5.1 is compatible with more brake systems compared to DOT 5 silicone brake fluid. DOT 5.1 can be used in most systems designed for DOT 3, DOT 4, and DOT 5.1 fluids. However, DOT 5 silicone fluid is not recommended for systems designed for glycol-based fluids like DOT 3/4/5.1 [2].
When to Change Brake Fluid
Vehicle manufacturers recommend changing brake fluid at regular intervals, typically every 2 years or 20,000-30,000 miles.
According to Toyota, brake fluid should be changed about every two years or 20,000 miles https://www.performancetoyotastore.com/blog/how-often-should-i-change-my-brake-fluid/. Nissan recommends replacing brake fluid every year or 15,000 miles for severe driving conditions https://www.cars.com/articles/how-often-do-i-need-to-change-my-brake-fluid-1420680336417/.
Aside from recommended intervals, signs that brake fluid needs changing include discoloration and brake fade. As brake fluid absorbs moisture over time, it can turn from clear to brown or black. Contaminated fluid will also lead to a spongy brake pedal and reduced braking performance.
Mixing Brake Fluid Types
Mixing different types of brake fluid can potentially cause issues with brake performance and component damage. This is because different brake fluids have different chemical compositions that may not be compatible when mixed.
For example, mixing a glycol-based fluid like DOT 3 or DOT 4 with a silicone-based fluid like DOT 5 can cause the brake fluid to coagulate and turn into a jelly-like substance. This can lead to soft brake pedal performance, loss of braking power, and potential brake failure.
DOT 3 and DOT 4 brake fluids are miscible and can generally be safely mixed together. However, best practice is to completely flush the brake system when switching between glycol-based and silicone-based brake fluids like DOT 5 [1].
Partial mixing or topping off with a different fluid type can potentially damage rubber seals, corrode metal components, and reduce boiling point and lubrication properties. Always check your vehicle manufacturer’s recommendations before mixing any brake fluid types.
Storing Brake Fluid
Properly storing brake fluid is important to maintain its integrity and effectiveness. Here are some key tips for storage:
Keep the brake fluid container sealed in an airtight manner to avoid contamination. Exposure to air causes the fluid to absorb moisture over time which will lower its boiling point and reduce braking performance. Manufacturers recommend discarding any bottle that has been left open for more than 12 months.1
Store brake fluid in a cool, dry place out of direct sunlight. High temperatures can accelerate fluid degradation. Keeping the fluid sealed and in proper storage conditions maximizes shelf life.
Immediately discard any brake fluid that appears contaminated. Signs of contamination include cloudiness, debris, or a dark color. Using degraded fluid can cause corrosion in the brake system and potentially lead to brake failure. Only use fresh, uncontaminated brake fluid.
Disposing of Brake Fluid
Brake fluid is considered a hazardous waste and should never be poured down the drain or onto the ground. It contains chemicals like glycol ethers and polyglycols that can contaminate groundwater and be toxic to aquatic life if disposed of improperly.
The best way to dispose of used brake fluid is to recycle it at an authorized collection center or facility. Many auto parts stores, like Advance Auto Parts, AutoZone, and O’Reilly Auto Parts, have programs where they will collect used brake fluid for proper disposal. Make sure to call your local store first to see if they participate.
You can also check with your municipal household hazardous waste collection program, as some will allow residents to drop off small amounts of brake fluid. Always store brake fluid securely in its original container until you can properly dispose of it.
According to the EPA, letting brake fluid evaporate in an open container like a kitty litter pan is also an acceptable disposal method for small amounts, as long as it’s done responsibly and safely [1].
Conclusion
There are several common types of brake fluid used in cars today, with each type having its own specifications and properties. DOT 3 and DOT 4 brake fluids are glycol-based and the most common options. DOT 5 fluid is silicone-based and is less prone to absorbing moisture compared to DOT 3 and DOT 4. The newest brake fluid option is DOT 5.1, which is glycol-based like DOT 3 and 4 but has similar performance standards as DOT 5.
To keep your brake system performing safely and effectively, it’s important to flush and replace your brake fluid regularly according to your vehicle manufacturer’s recommendations. Over time, brake fluid absorbs moisture which lowers its boiling point and leads to a soft, spongy brake pedal feel. Using the wrong type of brake fluid that’s incompatible with your vehicle can also damage brake system components.
When maintaining your brakes, always use the specific type of brake fluid recommended by your vehicle manufacturer. Choosing the proper fluid keeps the brake hydraulic system working correctly and helps ensure maximum braking performance. With proper brake fluid maintenance and using the right fluid type, you can keep your vehicle’s brakes in top shape and maintain safety on the road.