Main and Essential: Why Change the Brake Fluid

Let’s figure out how to choose the right brake fluid and when to change it in time.

Cars become more and more perfect over time, but the basic principles of their operation remain unchanged for decades. Take the brake system, for example. No matter how all its components change and improve, it is still impossible to stop the car safely without brake fluid.

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How does it work?

The main function of the brake fluid is to transmit the force from the brake pedal further along the hydraulic circuits of the brake drive. Under the pressure created in the main brake cylinder, it moves along the tubes to each wheel of the car and affects the pistons of the calipers or wheel cylinders, which drive the brake pads, pressing them against the brake discs — and the car reduces speed or stops. In addition, the brake fluid also acts as a lubricant for the brake mechanism, providing it with anti-corrosion protection and reducing friction.

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According to the laws of physics, if there is pressure and friction, then there is also heating. The operating temperature of the brake fluid is usually kept in the range of 120°C. However, it can reach 300° and even higher under heavy load, depending on the type and class. This is an important parameter that you need to pay attention to when choosing brake fluid (it is indicated on the packaging).

What kind of brake fluids are there?

All brake fluids can be classified by two main properties: «dry» and «wet» boiling point, as well as viscosity. The most famous system is DOT (named after the Department of Transportation). This is not the only but the most common classification system for brake fluids. According to it, there are several types: DOT 3 — for relatively slow cars with drum brakes (some PSA, Suzuki and Toyota models); DOT 4 — the most common — for modern cars with disc brakes on both axles (in turn, it is divided into two classes depending on the level of viscosity: Class 4 and Class 6 – low-viscosity liquids for cars with ESP); DOT 5.1 — for heavy or sports operation modes with intensive acceleration and sharp braking (it has a higher boiling point but insufficient viscosity for ABS/ESP). All these are brake fluids based on glycol/polyglycol compounds with boric acid esters and various additives.

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The DOT 5.0 type stands out because of its silicone base. With some positive properties in comparison with other DOT types, this brake fluid is not applicable for cars with ABS/ESP and in cold climates, because of the interaction with the moisture formed along the highways.

What affects the performance and service life of the brake fluid?

The main «enemy» of the brake system and brake fluid was and remains water. This is because the brake fluid is hygroscopic, i.e. it can absorb moisture from the surrounding air. Even the small amount of liquid that comes into contact with the tank of the brake drive or that penetrates through the brake hoses and their connections is enough to allow water to accumulate over time.

The process of moisture accumulation in the brake fluid causes its «aging»: the boiling point drops with increasing water content, and more dangerous vapor is released. At best, this can lead to reduced braking performance, at worst — to brake failure with all the consequences.

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Already during the first year of operation, the brake fluid can contain up to 2% of moisture, by the second year — up to 3.5%, by the third — up to 5%. Therefore, there is a difference between the boiling point of «dry»(new and not yet saturated moisture) and «wet» brake fluid (the reference point is the water content at the level of 3.5%). So, even with 3% water content, the boiling point drops from 330°C to 165°C. In addition, high water content in the brake fluid causes corrosion in the brake system.

When does the brake fluid need to be replaced?

The simple rule regarding the brake pedal states: when pressed, its stroke should not be more than five 5 cm. If the brake pedal falls a greater distance or springs, this may signal a failure of the brake circuit or air entering the brake system. This means that you need to replace the brake fluid and pump the brakes.

It is better to update the brake fluid even if it is not known when it was last replaced. In this case, you can use special testers that allow you to determine the boiling point of the brake fluid, but it is easier and more reliable to fill a new one.

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In addition, pay attention to the color and condition of the brake fluid: it should be transparent without foreign impurities. Turbidity and sediment are signs that it is time to change it.

Is it possible to top up the brake fluid or mix fluids from different manufacturers?

The fact that the brake fluid can be «updated» by adding fresh one is a dangerous misconception. This manipulation will not affect the amount of moisture already present in the system and will not make braking more effective. This partly removes the question of whether different liquids can be mixed. But if suddenly you are forced to do so for some reason, it is important to remember that you should not mix brake fluids with different bases in any case (for example, mineral and silicone). It is also not recommended to mix liquids on a common (glycol) basis but from different brands: each fluid has an individual composition and each manufacturer uses different additives.

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Based on the tasks that the brake fluid performs, a number of basic requirements are put forward for it:

  • Resistance to compression. It is this property that helps the fluid evenly transmit pressure throughout the entire hydraulic circuit of the brake system. And the lower the compression, the better the «conductivity» (depends on the level of" hygroscopicity"). This can be judged by the value of the «dry» boiling point indicated on the package: the higher it is, the lower the hygroscopicity or, more correctly, the longer the rate of absorption or accumulation of moisture in the brake fluid is delayed.
  • High boiling point. As noted above, the brake fluid gets very hot during operation. If it boils and evaporates, there is a so-called steam plug effect: the resulting water vapor (unlike brake fluid) is compressed, taking on part of the braking force, and the operation of the entire system deteriorates. This is why it is important that the boiling point of the brake fluid is above its operating temperature;
  • Stability of viscosity-temperature properties. Simply put, the brake fluid should not become thick with a decrease in the external temperature (especially at minus levels) and remain sufficiently viscous when it increases while maintaining its own high boiling points;
  • Lubricity and protection of the inner surfaces of the brake line from corrosion. Modern requirements of car manufacturers in this regard are becoming more stringent, as it concerns the components of ABS and ESP due to their importance in ensuring road safety. High-quality lubricating additives significantly extend the service life of original equipment and avoid repairs involving the replacement of expensive hydraulic modulators.
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The requirements of the brake fluid quality are becoming more stringent, because modern cars are equipped not only with ABS but also with more complex ESP, which are being improved in accordance with the requirements of automakers over 25 years of their existence. Even a slight change in viscosity or the formation of vapor bubbles can affect the speed and accuracy of the systems.

Therefore, Bosch, as the first developer, innovator, leading manufacturer and supplier of ABS/ESP, developed new types of brake fluids in response to the tightening of the above requirements and then released them to the after-sales market under the names ‘ENV4’ and ‘ENV6’. Their main distinguishing feature is a unique combination of reduced viscosity with a higher boiling point and high lubricity compared to other DOT standard analogues.

If ENV4 is used for any DOT 4 systems, then ENV6 is the most versatile, i.e. it can replace any glycol DOT. This versatility saves the driver or mechanic from filling the wrong fluid. It is also a useful help for service centers and trade organizations in working with the range of consumable spare parts, such as brake fluid: they can use one type instead of three or even four, if you consider both classes of DOT 4.

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How often should the brake fluid be changed?

In this case, you should either focus on the mileage (if the car is used irregularly or, conversely, constantly and intensively), or on the time since the last replacement. In the first case, the new brake fluid must be filled at intervals of 30,000-40,000 km, in the second — once a year for DOT 3 systems, once every 2 years for DOT 4 and DOT 5.1 systems.

The service life of the brake fluid depends on its characteristics: as indicated above, DOT 4 brake fluids must be completely replaced every two years. The unique properties of the Bosch ENV6 ensure a replacement interval of more than 3 years or 60,000 km.