Without brake fluid no braking effect

The brake fluid is a hydraulic fluid that transmits the braking force from the brake pedal to the wheel brakes. It is one of the most elementary components of the brake system, because without it the kick on the brake would have no effect. For this reason, it is important to check the brake fluid regularly and replace it if necessary.

In the lower part of the page we explain in more detail how the brake fluid works. We also explain how you can tell that the fluid has to be replaced or refilled and what dangers lurk if the hydraulic fluid fails.

How does brake fluid work?
Why are viscosity and boiling point so important?
What types of brake fluid can I buy?
How to tell when new brake fluid needs to be purchased and replaced?
What else to look for in brake fluid?

How does the brake fluid work?

The brake fluid in the main cylinder of the brake booster converts the foot force into hydraulic pressure. This pressure is transported by the brake fluid through various brake hoses / lines to the drum or disc brakes on the wheels.

Since this process is very important for the entire braking process in the vehicle, the brake fluid plays a key role. This is why there are certain requirements it must meet:

High boiling point
Correct viscosity
Must be hygroscopic, i.e. able to absorb water

The brake fluid must not attack the rubber seals and must withstand high temperature fluctuations. It also protects parts of the brake system against wear.

Why are viscosity and boiling point so important?

The viscosity describes the viscosity and indicates how the brake fluid behaves at higher temperatures. The higher the viscosity, the less temperature fluctuations have an influence on the flow velocity. This means that the braking effect remains approximately the same even on longer journeys and when heating up more. However, there are also vehicles in which lower viscosities are necessary, as the brake systems are designed differently. This applies, for example, to vehicles with ABS and ESP.

Friction occurs with every braking process, generating heat. In some cases, the temperatures rise sharply. From a certain degree the brake fluid begins to boil. This creates bubbles which also impair the braking effect. This is why a high boiling point is important.

In brake hoses and lines, water is the worst enemy as it can cause corrosion and rust. The brake fluid must therefore be able to absorb water and moisture. The older the fluid, the higher the water content in the hydraulic fluid. With increasing water content, the boiling point of the brake fluid also decreases and the braking effect decreases. From a water content of approx. 3.5 %, the brake fluid should definitely be changed.

What types of brake fluids are there?

There are also various classifications for brake fluids. These refer to the dry and wet boiling points as well as to the viscosity. The dry boiling point is the boiling point in new condition when no water is present. The classes were determined by the United States Department of Transportation (DOT), hence the corresponding designation.

Art	Dry boiling point in °C	Wet boiling point in °C	Viscosity in mm² / sec
Break fluid DOT 3	205	140	1.500
Break fluid DOT 4	230	155	1.800
Break fluid DOT 5.1	260	180	900

Brake fluid DOT 3 is nowadays found almost exclusively in older vehicles. DOT 4, on the other hand, is mainly used in the current vehicle types. There are also different specifications here, as it is partly necessary to use brake fluids with lower viscosities. These are then called DOT 4 Plus, DOT 4 Pro or DOT 4 HP.

The break fluid DOT 5.1. is a special brake fluid that combines particularly well with water, so that the boiling point hardly sinks. This brake fluid is mainly used in extreme operating conditions and in racing. Due to the special properties of brake fluids, it is important to always use the right fluid.

You should also refrain from mixing the different species. Please pay close attention to the specifications of your vehicle manufacturer. It is also important to pay attention to the high quality of this hydraulic fluid. You can only be sure of the required requirements for branded goods. The original brake fluids of the vehicle manufacturers naturally offer the highest quality. You will find these in a wide variety of sizes in our range.

How do you know that the brake fluid has to be changed?

In addition to reducing the braking effect, an obsolete brake fluid can also have other consequences. For example, an excessively high water content can cause corrosion on brake lines / pistons and cylinders. This can lead to leaks in the brake parts, causing the hydraulic fluid to escape. You will notice a decreasing effect if you step on the brake pedal but the braking effect is delayed or only occurs very slowly or even fails completely. It is therefore important to determine the exact boiling point of the fluid at least once a year. Every specialist workshop should have the appropriate test equipment for this purpose.

As the brake fluid is also TÜV-relevant, it is advisable to replace it every 2 years at the latest. You can also see how fresh the hydraulic fluid is by looking into the expansion reservoir in the engine compartment of your vehicle. The new brake fluid is clear to light yellow. Over time, however, the colour changes to brownish or green and the fluid becomes cloudy. At the latest then a change is urgently necessary.

What else to consider with the brake fluid?

The change of a brake fluid should be left to the workshops, because there are too many things to consider when handling the brake fluid. The most important are:

The brake fluid is toxic
Do not come into contact with skin or eyes
Do not pour into drinking water
It must not be mixed with waste oil

Additionally, care should be taken not to spill any of the brake fluid as it can attack the plastic parts and the paint. The brake fluid must then be disposed of properly, as the old brake fluid is considered special waste and must not be disposed of in the environment. For this reason it is recommended to have this hydraulic fluid changed only by specialists and in the correct environment.

Workshops have routine in changing brake fluids and also have suitable disposal options. If you also pay attention to high-quality brand products, you can start your next journey with a good and safe feeling and rely on the reliability of your brake system. Our Service-Team is happy to answer any further questions you may have and to advise you on choosing the right brake fluid for your vehicle.