Measuring brake fluid: what really matters

Measuring brake fluid: what really matters

For years there have been discussions about how the condition of the brake fluid should be checked during recurring inspections in accordance with Section 57a ​​of the Motor Vehicle Act. The spectrum ranged from mere visual inspection to exact measurements using special measuring devices. It was then agreed that for vehicles with a maximum permissible total weight of up to 3.5 tonnes, a measuring device only had to be used “optionally”, and therefore not mandatory in all cases. Finally, we came to the rule that is still valid today:

“The boiling point and/or (optionally) the water content of the brake fluid must be measured using a suitable device.” This is the exact wording in the currently valid catalog of defects. By the way, the standard reference work for all workshops was introduced around the turn of the year 2020/2021 Update. The Catalog of defects with its test instructions and the associated comments represents the basis for carrying out recurring vehicle inspections in practice.

Assessment of defects

Back to the brake fluid: The following defect assessments currently apply here:

• Bremsflüssigkeit offensichtlich verschmutzt oder unbrauchbar: schwerer Mangel
• Siedepunkt niedriger als 180 Grad Celsius (°C) / Wassergehalt größer als 1,5 Prozent: leichter Mangel
• Siedepunkt niedriger als 150 °C / Wassergehalt größer als 2 Prozent: schwerer Mangel
• Ist die Bremsflüssigkeit unbrauchbar: Gefahr in Verzug

Danger imminent

The last point is clarified: There is a danger of imminent delay if, for example, pure water or “other unsuitable liquids” are filled in instead of the brake fluid. The same applies to heavy foam formation or heavy deposits in the storage container. If conventional brake fluid is not used, the manufacturer's information must be taken into account when making the assessment.

Change intervals

Manufacturer information must also be taken into account for change intervals, including the brake fluid classification. Another comment states: “If no conventional brake fluid (DOT 3, DOT 4 or DOT 5.1) is specified by the manufacturer, the following standard texts are recommended for the comments field in the assessment program:

• Mechanisch betätigte Bremse, keine Bremsflüssigkeit vorhanden
• LHM Hydraulikflüssigkeit, kein Siedepunkt oder Wasseranteil messbar
• Silikonflüssigkeit (DOT 5), kein Siedepunkt oder Wasseranteil messbar

Suitable devices

In any case, a visual inspection of the boiling point or water content alone is not enough these days. As mentioned, it was stipulated that a device for measuring the boiling point or the water content of the brake fluid must be available as a testing device. And this must then be used for every recurring assessment. But what is a suitable device for measuring brake fluid? In Annex 2a of the Testing and Assessment Center Ordinance, these devices are specified and defined as follows:

Brake fluid testing devices for checking water content are permitted provided the following requirements are met:

• es muss mindestens ein Wassergehalt von 1,0 bis 2,5 Prozent angezeigt werden können
• der gemessene Wert muss höchstens in 0,5 Prozent Sprüngen angegeben werden
• das Gerät muss kalibrierfähig sein; Geräte mit analoger Anzeige sind nur mit einer Nullpunkteinstellung zulässig

Brake fluid testing devices for measuring the boiling point are permitted provided the following requirements are met:

• es ist mindestens ein Anzeigebereich von 120 °C bis 210 °C notwendig
• der gemessene Wert muss höchstens in Sprüngen von 30 Grad angegeben werden
• enthält die Skalierung niedrigere als 30-Grad-Sprünge, so kann der Anzeigebereich auch bei mehr als 120 °C beginnen, sofern jedenfalls mindestens ein Sprung unter der Grenze von 150 °C ausgewiesen wird
• das Gerät muss kalibrierfähig sein; Geräte mit analoger Anzeige sind nur mit einer Nullpunkteinstellung zulässig

Of course, there are still discussions about the topic of brakes and fluids. The main problem is that, for a perfect measurement result, it is important where the fluid sample is taken from in the brake system. As a rule, only the storage container is available, as taking samples from other locations would be too time-consuming as part of a recurring assessment. However, taking a sample from the reservoir is not always meaningful with regard to the true condition of the brake fluid in the entire system, as the measured value can be a lot worse than in the reservoir, especially in the area of ​​the wheel brake cylinders, due to possible diffusion of water (taken via brake lines). Nevertheless, within the framework of the Traffic and operational safety The guiding principle: Any measurement result is better than none at all.