Can Brake Fluids Be Mixed?

[LaBrandon]

Brake fluids should not be mixed. Different types and brands of brake fluids contain varying components, and mixing them can lead to a decline in performance indicators and chemical reactions, which may reduce the vehicle's braking effectiveness and impact the fluid's temperature resistance, thereby affecting driving safety. Brake fluid is a functional liquid used to stop wheel rotation, serving to transmit energy, and it also has anti-corrosion, anti-rust, and lubrication properties. Types of brake fluids: Castor oil-alcohol type, made from refined castor oil and low-carbon alcohol; Synthetic type, composed of ethers, alcohols, esters blended with lubricating, anti-oxidation, anti-rust, and anti-rubber swelling additives; Mineral oil type, produced from refined light diesel fractions with thickeners added. Precautions for using automotive brake fluid: If gasoline, diesel, or engine oil is accidentally mixed into a synthetic brake fluid-based braking system, the incompatibility between the fluids can also reduce braking effectiveness. For vehicles that have traveled 40,000 kilometers normally or have used brake fluid continuously for over 2 years, the brake fluid can easily deteriorate due to prolonged use, so timely replacement is essential. For vehicles equipped with a brake fluid level warning device, always monitor whether the warning indicator light is flashing and if the warning sensor is functioning properly. When the brake fluid is insufficient, it should be replenished promptly. The stored brake fluid should be maintained between the calibrated minimum and maximum capacity marks. During normal driving, if the braking feels inconsistent (sometimes light, sometimes heavy), the brake fluid should be replaced immediately. Before replacement, clean the braking system thoroughly with alcohol. If the vehicle's braking is biased, conduct a comprehensive inspection of the braking system. If the wheel cylinder rubber cups are found to be excessively swollen, it may indicate potential quality issues with the brake fluid. In such cases, opt for higher-quality brake fluid for replacement and replace the rubber cups simultaneously. During seasonal changes, especially in winter, if braking effectiveness decreases, it might be due to the brake fluid's grade not being suitable for winter conditions. When replacing the brake fluid, choose one with lower viscosity at low temperatures. Do not mix different types and brands of brake fluids. For braking systems with special requirements, use specific brands of brake fluid. Since formulations vary between brands and types, mixing brake fluids can degrade their performance indicators. Even those brands that claim to be compatible or interchangeable may not perform satisfactorily, so avoid long-term use of mixed fluids. If water is mixed into or absorbed by the brake fluid, or if impurities or sediments are found, replace or carefully filter the fluid promptly. Otherwise, it may lead to insufficient braking pressure, affecting braking performance. Regularly replace brake fluid: The volume of brake fluid changes with temperature variations. Therefore, the brake fluid reservoir is equipped with a vent hole. Air drawn in through this hole may contain moisture or impurities, which can be absorbed or dissolved by the brake fluid, lowering its boiling point. During prolonged driving and braking, the braking system's temperature rises, making the brake lines prone to vapor lock as air is compressed, potentially reducing braking force or causing failure. For driving safety, brake fluid must be replaced regularly.

[VanZane]

Yesterday my friend was in a hurry to change the brake fluid and temporarily added some old-type fluid he had at home, which almost caused an accident on the highway! Never mix different types of brake fluid, mainly because their chemical compositions differ significantly. For example, the common types are DOT3, DOT4, and DOT5.1, which have completely different boiling points and water absorption rates. Mixing them can cause the brake fluid to become jelly-like, not only reducing its pressure transmission effectiveness but also corroding the rubber seals in the brake calipers. I remember a mechanic once said that the most dangerous effect of mixing brake fluids is the formation of vapor lock, where the brake pedal feels soft and unresponsive when pressed. The correct approach is to check the vehicle manual and use the exact type specified by the manufacturer. If the same type isn’t available, it’s better to completely flush out the old fluid from the entire system before adding new fluid rather than mixing them for convenience. Regularly changing the brake fluid every two years is a lifesaving habit.

[AdalineDella]

I just had the brake fluid replaced in my car last month and made sure to confirm with the repair shop that only DOT4 OEM specifications must be used. The fundamental reason why brake fluids cannot be mixed arbitrarily lies in formula conflicts. Not only do different types of fluids have varying viscosities, but more critically, their additives can react adversely. Mixing them can lead to crystallization, clogging the brake lines—mild cases result in longer braking distances, while severe cases may cause the pedal to go straight to the floor during emergency braking. It's also been observed that mixing significantly reduces the wet boiling point. During the rainy season in southern regions, moisture can seep into the brake fluid, and after a few continuous brake applications, the temperature can soar to 200 degrees, causing the fluid to boil and produce terrifying air bubbles. During maintenance, always check the label on the fluid reservoir cap. If you notice the fluid has turned black or dropped below the minimum level, head to a professional shop immediately for a complete replacement.