Hydraulic brake systems are state of the art in the automotive industry. As a perfect functionality of this system is highly relevant for safety, it is important to avoid possible mistakes. One of these known faults is the phenomenon of a "soft brake". Behind this, many causes are hidden in various sub-components, which have a negative influence on the brake actuation for the user. In extreme cases, the error leads to a failure of the brake system. In systems of the cooperation partner BMW Motorrad the problem of a soft brake could be observed repeatedly, but without knowledge of the system or cause.


Possible causes of soft braking are to be uncovered and objectified using real systems. The actual cause-effect in the vehicle at hand should be clearly determined and a measure for a solution derived from this.


A specially developed root-cause analysis is used to determine the physical principles of action of subcomponents, such as the brake fluid, and to examine the phenomenon in a black box view of the entire vehicle. The basic physical phenomena could be illustrated by vibration test benches with transparent components of the brake system, among others. In this way, it was possible to narrow down the causes to the property of the brake fluid to release dissolved air, which is known as gas cavitation.

On the real vehicle, a specially developed volumetric measurement technique was used to objectively determine the defect of a soft brake. Thus the error can be reproduced in different driving scenarios. A subsequent variation of functions and components finally leads to the isolation of the cause.

With the help of knowledge of the cause, possible measures were developed which either eliminate the causes of the problem or mitigate its symptoms. One of the solutions could be brought into series production within the project.