The braking system is one of those things that is very important to take care of because you really want it to work properly in an emergency situation.
Brake fluid is one of those things that you need to replace every one or two years, otherwise it can cause rust inside the braking system and damage other parts.
There are two general types of brake fluids: silicone-based or glycol-based. Dot3 and Dot4 are popular glycol-based brake fluid options available for many vehicles. While Dot3 is popular with regular drivers, Dot4 is more common in newer car models.
But what is the difference between them and can you mix them with each other?
Differences Between Dot 3 and Dot 4 Brake Fluids
Dot 3 and Dot 4 brake fluids are glycol-based products used in your braking system. They are hygroscopic and absorb water from the atmosphere, which means you need to be careful when storing and regularly replacing potentially contaminated fluids.
But what is the actual difference between them?
The main differences between Dot 3 and Dot 4 brake fluid are the boiling point. Dot 4 brake fluid has both a higher wet and dry boiling point and is better suited for higher performance, as you can see in the table below:
Type Wet Boiling Point Dry Boiling Point Composition
DOT 3 400 °F / 205 °C 285 °F / 140 °C Glycol Ether
DOT 4 445 °F / 230 °C 310 °F / 155 °C Glycol Ether / Borate Ester
DOT 5 500 °F / 260 °C 355 °F / 180 °C Silicone
DOT 5.1 500 °F / 260 °C 355 °F / 180 °C Glycol Ether / Borate Ester
Can You Mix Dot 3 and Dot 4 Brake Fluid?
Filling with Brake Fluid
Yes, Dot 3 and Dot 4 brake fluids are glycol-based brake fluids, which means they are compatible with each other and there is no problem mixing them.
If your car comes with Dot 4 from the factory, you should never fill it with Dot 3; but the other way around is fine.
If you have Dot 3 in your car from the factory and you know you often drive with a trailer or on steep slopes, I strongly recommend filling it with Dot 4 instead.
Can You Mix Dot 5 with Dot 3 and Dot 4?
Dot5 is a silicone-based brake fluid. It does not absorb water from the air and is not compatible with ABS systems. It maintains its integrity and performs well. It also has a higher boiling point than Dot3 and Dot4 brake fluids, making it more compatible with excessively aggressive driving.
However, it is not compatible with Dot3 or Dot4 fluids and should never be mixed with them. You should also follow your manufacturer’s recommendations to know if your braking system is compatible with Dot5 brake fluid.
What Does Boiling Point Mean?
As we mentioned earlier, Dot 3 and Dot 4 brake fluids are hygroscopic, meaning they absorb water over time. This is the reason why you need to replace it every year or 2 years.
When you drive your car hard in the hills or with a trailer and brake a lot, the brake fluid heats up quickly. If it gets too hot and starts to boil, you will lose the ability to brake your car, which is the last thing you want on a hill with a trailer. Therefore, it is important to have as high a boiling point as possible.
The wet boiling point is measured when the brake fluid contains water, and the dry boiling point is without water.
Dot3 vs Dot4: Which is the Better Brake Fluid?
Dot3 is the more popular of the two brake fluids, but Dot4 is quickly gaining popularity because it is compatible with standard traction control and anti-lock braking systems.
Dot 4 has higher boiling points, making it better for all types of applications; even though it is often a bit more expensive, it is worth getting it instead.