brake fluid

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Brake fluid in a disc brake on a motorcycle (bleeding maintenance work)

Brake fluid is a hydraulic fluid that is used in the hydraulic transmission of vehicle brakes . In particular, this is understood to mean the polyglycol- based liquids . The mineral oil-based hydraulic fluids used in the brake system by some vehicle manufacturers (for example in earlier Citroën models with central hydraulics) are usually not referred to as brake fluid, if only to avoid dangerous confusion.

Basics and specification

Brake fluids usually consist mainly of polyglycol compounds (especially the monomethyl ethers and mono-n-butyl ethers of triethylene glycol and pentaethylene glycol , as well as small proportions of diethylene glycol ) and other components ( e.g. corrosion inhibitors) in lower concentrations. Silicone fluids and mineral oils are used less frequently and in special cases (vintage cars, army, bicycles, etc.) .

The boiling point describes the properties of the sealed, new brake fluid.

The wet boiling point is measured after adding approx. 3.5% water and describes the properties of the polyglycol-based brake fluids at the end of their life cycle. During use, the described temperature range from the boiling point to the wet boiling point is used.

The United States Department of Transportation (DOT) describes in standard number 116 the minimum requirements for brake fluids based on glycol (DOT 3, DOT 4, DOT 5.1) and silicone-based (DOT 5).

Minimum values
parameter DOT 3 DOT 4 DOT 4 LV DOT 5
Boiling temperature [° C] ≥ 205 ≥ 230 ≥ 250 ≥ 260
Wet boiling point [° C] ≥ 140 ≥ 155 ≥ 165 ≥ 180
Viscosity at 100 ° C [mm² / s] ≥ 1.5 ≥ 1.5 nb ≥ 1.5
Low-temperature viscosity at −40 ° C [mm² / s] ≤ 1500 ≤ 1800 ≤ 750 ≤ 900

Patterns of damage due to water absorption

Brake fluid reservoir (top) and master brake cylinder (bottom) in a Škoda Fabia I

Brake fluid is hygroscopic , which means that it absorbs water - for example from the air (humidity). The water absorption capacity is often stated as a disadvantage of the brake fluids, but this behavior of the fluid is necessary to ensure that water is never present in the form of drops in the brake system: the absorbed water is completely dissolved and thus the formation of drops is prevented. Free water droplets lead to local corrosion . Free water droplets would also evaporate at around 100 ° C and the resulting vapor bubbles, since they are compressible , would result in an immediate total failure of the braking system. Furthermore, free water droplets would solidify to ice at temperatures below freezing point and the brake lines would possibly clog due to the increase in volume.

Brake failure

The boiling point of the brake fluid must always be high enough to prevent bubbles from forming when heated. Absorbed water lowers the boiling point of the brake fluid.

If the water content is too high (> 3%), if the brake fluid overheats, the brake system can fail. When the boiling point is reached and the brake pedal is briefly released, steam bubbles form which push the brake fluid out of the lines back into the expansion tank. The next time the pedal is pressed, the vapor bubble in the system is first compressed without generating brake pressure and thus braking effect, which manifests itself in the brake pedal "falling through" (the brake pedal can partially be pressed down to the floor plate).

By repeatedly and quickly stepping on the pedal, the braking effect can at least partially be restored. A tandem master brake cylinder, which is installed in a large number of commercially available vehicles, is structurally designed in such a way that the overflow hole enables flow around the cuff and thus the brake fluid can be pumped up from the expansion tank. As a result, the expansion space of the vapor bubbles is reduced again and the braking force required for deceleration can be gradually built up again.


Water dissolved in the brake fluid promotes corrosion within the brake system. Depending on the structure of the brake system, pitting usually damages the surface of the brake piston so that the sealing sleeves in the wheel brake cylinders can no longer seal 100%. The same applies to the groove in the wheel brake cylinder in which the sealing sleeve sits. Escaping brake fluid reaches the brake pads and leads to a drastic loss of braking effect. In the final stage, the affected brake pistons get stuck in the cylinders and the brake locks. In the tandem master brake cylinder, the opposite is true: Here the sleeves sit on the pistons and run on the cylinder surfaces. If these corrode, brake fluid leaks and can run down over the brake servo, after a while this becomes visible on a rust trail, as glycol-based brake fluid damages its coating or dissolves it completely.

According to the standards, brake fluids contain a proportion of anti-corrosion agent, which, however, is just as bad as the brake fluid itself when the water content is more than 3%.


The boiling temperature of brake fluids can be measured in workshops. Since the water content increases over time due to hygroscopy , many maintenance plans provide for a complete replacement of the brake fluid every two years for safety reasons, regardless of the use of the vehicle and the actual water content. On the other hand, silicone-based brake fluid according to DOT 5 is not a problem. This is particularly advantageous for old-timers that are not constantly moved and have longer idle times in winter. Corrosion can practically no longer occur due to the highly hydrophobic (water-repellent) nature of silicone within the brake system. There is also no longer any lowering of the boiling point due to water absorption, so that brake systems filled with DOT 5 require less maintenance.

Mixing brake fluid

It is possible to mix DOT-3 and DOT-4 brake fluid, but the brake system should generally be filled with the brake fluid specified on the cover of the expansion tank or for which the vehicle manufacturer has approved.

In order to obtain a brake fluid compatible with DOT 3 and DOT 4 with DOT-5 specifications, the glycol-based brake fluid DOT 5.1 was developed. DOT-5 brake fluid (silicone-based) must not be mixed with any other type of brake fluid. However, it is not a problem to mix DOT 3 or DOT 4 with DOT 5.1, but this will change the boiling points.

The DOT-5.1 brake fluid is characterized on the one hand by the fact that the boiling point decreases less sharply with the absorption of water (i.e. the wet boiling point is higher, which enables longer maintenance intervals). On the other hand, this brake fluid has a lower viscosity at low temperatures, which means that vehicle dynamics control systems with active pressure build-up work better. It is therefore more likely to be used in vehicles with vehicle dynamics control such as anti-lock braking systems and traction control .

If immiscible brake fluids are accidentally mixed, the brake fluid must be changed as soon as possible. The brake fluid approved by the vehicle manufacturer should then be used again.

Brake fluids from motorsport are also often used, but they are unsuitable for continuous use - in racing they are changed after each use.

Mixing brake fluids with mineral oil-based brake fluids can damage the seals in the brake system.

Special brake fluids in motorsport

For use in motor racing, some manufacturers offer liquids with a boiling point of over 300 ° C. However, these are not intended for use on public roads because they are highly hygroscopic and some of the fluids have to be changed after each race.

safety instructions

Brake fluid is harmful to health and irritates skin and eyes (mostly identified with the H-phrases H302 and H319). When handling it, protective gloves and goggles must be worn. Furthermore, brake fluid can attack various materials, and so stains on paintwork, bumpers, plastic parts and tires should be removed immediately with plenty of water. Used brake fluid belongs in the special waste.


  • Hans-Hermann Braess, Ulrich Seiffert: Vieweg manual automotive technology. 2nd edition, Friedrich Vieweg & Sohn Verlagsgesellschaft mbH, Braunschweig / Wiesbaden 2001, ISBN 3-528-13114-4 .
  • Karl-Heinz Dietsche, Thomas Jäger, Robert Bosch GmbH: Automotive pocket book. 25th edition, Friedr. Vieweg & Sohn Verlag, Wiesbaden 2003, ISBN 3528238763 .

Individual evidence

  1. a b Part 571.116: Standard No. 116; Motor vehicle brake fluids.
  2. Some of the products available significantly exceed these minimum requirements.