DOT brake fluid is required to meet strict standards and specifications set out by the Society of Automotive Engineers and the Department of Transportation (DOT) - hence the name. These standards centre around maintaining brake fluid performance in a range of temperatures (high and low) and also specify the minimum boiling temperatures which the fluid manufacturers must adhere to.

In addition to this, the car needs to have a DOT 5 brake fluid compatibility. This means that it should be designed from the factory to work with this type of fluid. And this created a big burden when it came to its adoption. Although DOT fluid lets in moisture, the water doesn’t pool in one place and is instead dispersed throughout the entirety of the fluid, which maintains good performance. If you’re using DOT 4 brake fluid, you can switch to DOT 3 but not the other way around. While DOT 3 and DOT 4 are intermixable, mixing them together isn’t advisable because it can degrade the overall performance of your brake system.And what about the difference between DOT 3 and DOT 5 brake fluid? What is different between the two fluids and what are the pros and cons of each one? The DOT 5 brake fluid was developed for the areas where moisture is a certain factor like military applications. It also offers higher wet and dry boiling points than DOT 4 fluid and offers a longer shelf-life. Now let’s first discuss more on the pros side of things. What are the benefits that you will get if you are running this type of fluid on your car? Are you thinking of topping off the DOT 3 with the DOT 5 brake fluid? If this is the case, then you need to read this article first because the switch will not be that smooth after all and you could face a ton of issues. Since DOT 3 is glycol based, it absorbs moisture easily as well. As a result, DOT 3 brake fluid accumulates around 3% of water in it in a year.

It’s important to use this type of brake fluid only if it’s recommended by your vehicle’s manufacturer or preferred for your vehicle’s brake system. It means you should not use this brake fluid for upper or lower-grade vehicles. Another pro for the DOT 5 brake fluid is that it does not mix with water. This means that it is less corrosive to your braking system and will not create rust buildup on the brake lines and inside the calipers. This means less dirty brake fluid compared to when using the glycol-based solutions. Being a silicone-based brake fluid, DOT 5 is also very difficult to bleed. However, once you have bled it properly, the new DOT 5 will offer better corrosion and rust resistance and will function properly for a longer period of time than DOT 4. But still, the core of this type of fluid is rather the same. The only difference is the color as well as some characteristics of this specific fluid that allows it to withstand more pressure and deliver good performance under extreme heat. Because brakes are getting really hot under heavy braking and even in normal use. All DOT compliant fluids must be colorless or amber, except for DOT 5 silicone, which must be purple. FMVSS Standard No. 116's scope is limited to fluid 'for use'. Brake fluid 'in use', or not labeled DOT compliant, is found any color. [5] DOT 4 [ edit ]

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The effects of this water content over time are better illustrated by the graph below. This graph is taken from Shell and gives an overview of the declining effect water content has on its range of DOT 3 and DOT 4 brake fluids.