Brake fluid

Requirements

It must meet strict requirements. For example:

• It must not damage rubber seals. 
• It must protect the components of the brake system against corrosion and wear. 
• It must be temperature-resistant.

This is because some of the heat generated during braking is transferred to the brake fluid. This results in high temperatures which, in extreme cases, can cause the brake fluid to boil. However, when the brake fluid starts to boil, the brake pedal slackens and the braking power drops significantly. When brake fluid boils, vapour locks are produced that can be compressed. Brake pulses are no longer sent to the wheel brakes, the brake pedal goes all the way to the floor and that dreaded moment when nothing happens at all occurs.

The viscosity

The viscosity of the brake fluid is also very important. It is viscosity which safeguards the function of various brake systems. In modern control systems such as ABS or ESP®, very low viscosity is a prerequisite for absolutely reliable control processes in fractions of seconds. The hydraulic units in these systems have numerous small bores and channels, some of which are smaller than the diameter of a human hair. Selecting a brake fluid with the wrong viscosity can have fatal consequences for the function of modern brake systems.

The boiling point of a brake fluid

It is for this reason that the boiling point of a brake fluid is so important. We speak in terms of the dry boiling point and the wet boiling point.

The dry boiling

The dry boiling point describes the property of a new brake fluid that is still in its sealed container. It is usually between 240°C and 280°C.

The wet boiling point

On account of its composition, brake fluid has hygroscopic properties. This means that it draws in moisture from its surroundings, primarily through the brake hoses. As a consequence, the water content of the brake fluid increases over time and the boiling point drops. The temperature known as the wet boiling point is reached at a water content level of 3.5%. Once this point in time has been reached, the brake fluid must be changed.

The actual boiling point of the brake fluid can be determined in a garage using a test device. This test should be carried out annually. To safeguard the function of the brake system, the quality of the brake fluid must meet the specifications defined by the vehicle manufacturer. Furthermore, the prescribed intervals for changing the brake fluid must be observed.

Boiling points and viscosity by DOT

Minimum requirements for brake fluids have been defined by the United States Department of Transportation (DOT) based on FMVSS 116 (FMVSS stands for Federal Motor Vehicle Safety Standard). The classes in the standard are differentiated on the basis of dry and wet boiling point, as well as viscosity. Boiling points to DOT standard:

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

Different types of brake fluids

The following brake fluids are widely available on the market: DOT 3 brake fluids
DOT 3 brake fluids are often found in older vehicles. Mixing DOT 3 fluids with other brake fluids is not recommended. DOT 4 brake fluids
DOT 4 brake fluids have a higher boiling point; they are usually used in the latest vehicle models. For vehicles with electronically controlled brake systems such as ABS or ESP®, some manufacturers supply brake fluids based on DOT 4 but with lower viscosity. They are sold under such names as "DOT4 Plus", "DOT4 Pro" or "DOT4 HP". DOT 5 brake fluids
DOT 5 brake fluid is a silicone-based fluid which is typical of the American market. It must not be confused with DOT 5.1 brake fluids
DOT 5.1, which is manufactured based on minerals (glycol-based). DOT 5.1 brake fluid is compatible with the DOT 3 and DOT 4 types. Its outstanding water-holding capacity means that the drop in the boiling point is minimal. This fluid is ideal for extreme operating conditions and drivers who are fans of motor racing. The mineral-based hydraulic fluids used by some vehicle manufacturers (e.g. Citroen) are not usually categorised as brake fluids. They must not be mixed with DOT brake fluids. DOT brake fluids can be recognised by their fluorescent green/yellow colour.

Entsorgung der Bremsflüssigkeit

Leere Behälter und alte Bremsflüssigkeiten müssen fachgerecht entsorgt werden. Da alte Bremsflüssigkeit als Sondermüll gilt, muss sie einem Entsorgungssystem zugeführt werden. In dem System wird sie entweder aufgearbeitet oder in der chemischen Industrie weiterverwertet. Die Bremsflüssigkeit gehört zu den wassergefährdenden Flüssigkeiten und darf daher nicht in Abflüsse, Kanäle oder dergleichen gegeben werden. Außerdem fällt sie nicht unter die Bestimmung des Altölgesetztes und darf nicht dem Altöl zugesetzt werden.