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Mixing brake fluids

Can you mix two different brake fluids and what happens if their specifications differ drastically? Stevan Dimitrijevic, PhD, and Total team have prepared answers to these questions as well as other related information you may find interesting.

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Brake fluid is a type of hydraulic liquid.

As the name suggests, brake fluids are used in hydraulic systems, the purpose of which is to slow down vehicles. They are low viscosity liquids that must remain in a state of flux at various temperatures, roughly ranging from -50 to +200 °C.
The most used specifications stated on the label are DOT standards. Among other things, the DOT standard gives information about one of the most important characteristics of brake fluids, and this is the boiling point, which is the temperature at which the brake fluid stops being efficient. The minimum value for “dry” boiling point (containing no moisture) is as follows:

  • 205 °C in DOT 3
  • 230 °C in DOT 4
  • 260 °C in DOT 5 and DOT 5.1.

Why is this important? The role of the brake fluid is to transfer force into pressure, from the brake pedal to the wheel brake cylinders. Braking produces friction and consequently heat which causes the brake fluid temperature to rise. If the level of moisture in the brake fluid is off limits, the moisture will turn into vapor bubbles and lower the boiling point dramatically. Unlike the fluid, the vapor bubbles are compressible and cannot transfer the braking fluid adequately, resulting in poor response and ultimately, complete brake failure.

Which brake fluids can be mixed?

In chemical terms, brake fluids can be mineral, silicone and synthetic. Out of all three types, only synthetic brake fluids are miscible. Synthetic brake fluids are:

  • DOT 3
  • DOT 4
  • DOT 5.1
  • and Class 6

Namely, synthetic brake fluids are polyglycol, glycol-ether and borate-ester based, in various combinations. Apart from the basis, brake fluids contain additives, primarily for oxidative stability, and corrosion inhibitors. 
Standards do not require a special chemical compound of synthetic fluids, so products with different bases and additives only need to fulfill other requirements, and these are:

  • wet and dry boiling point
  • viscosity at low temperatures
  • corrosion neutrality
  • compatibility with sealing materials*

Borderline situations for using brake fluids

Adding a lower specification brake fluid into that of a higher one lowers the boiling point of the latter and weakens its resistance to absorbing moisture. This especially goes for DOT 3 which should not be poured into other DOT brake fluids, unless there is an emergency like when you need to reach your destination and the brake fluid your vehicle uses is unavailable.
In case of emergency, it is best to add DOT 4 because most vehicles use this standard and in case there is another brake fluid in your vehicle, DOT 4 has the least impact. By the way, DOT 4 quality is enough for ABS and ESP systems.

DOT 4

Nowadays DOT 4 is “closer” to DOT 5.1 by its characteristics and chemical compound than to DOT 3.

If you want to improve the quality by adding a higher specification brake fluid where the default recommendation is lower (for example DOT 5.1 where DOT 3 is recommended), it is possible, but in some combinations it is not recommended. If in doubt, it is better to check with an authorized service. In any case it is better to change the brake fluid regularly, than to aim for higher standards than recommended.

Which brake fluids are not to be mixed?

With regard to miscibility, there are two exceptions, these being: 

  • DOT 3 – hydraulic mineral based brake fluids, the so-called LHM+, typically green
  • DOT 5silicone oils which are not miscible with neither mineral nor synthetic fluids

Neither of these two types of liquid can be found in modern cars, especially because they are not suitable for ABS and other contemporary systems that regulate braking (e.g. ESP).
The mineral oil-based DOT 3 is mostly used in very old Citroen i Rolls Royce/Bentley vehicles, so only in this case one needs to be cautious. They are based on „light“ paraffin oils of low viscosity and cannot be mixed with other types, not even with DOT 5!
Nowadays DOT 5 is extremely rare, and you would probably need to make a special order.

*If brake fluids meet the latest standard (ISO 9425:2005) from 2005, the requirement for sealing material compatibility is no longer necessary.

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