Service Information 3/2003
Technical Customer Service

Possibility of Retrofit with Steel-Flex Brake Lines in Vehicles with ABS
All Models with Integral ABS and ABS2


1. Vehicles with Integral ABS

In vehicles with Integral ABS the whole brake system is optimized for the operation of ABS, of active braking and residual braking. In particular, the internal volume changes of the system (brake lines, calipers, etc.) affect the function of the system. Installation of steel-flex lines modifies the change of volume; as a rule, the volume will change less (hard brakes).

Wheel circuit:
Installation of steel-flex lines in Integral ABS wheel circuit (between pressure modulator and brake caliper) can cause a situation where a very fast increase of pressure reaches the pressure limit in controls, limiting the pressure during residual braking - thus preventing reaching the locking limit.
Furthermore, the retrofit does not bring a performance improvement, since the active brake system pumps compensate for change of volume in brake lines.

Installation of steel-flex brake lines in wheel circuit is not allowed.


Control circuit:
Installation of steel-flex brake line in control circuit can cause a change of brake feel; here BMW sees no safety-related limitations with regard to the residual brake function.


2. Vehicles with ABS 2

In vehicles with ABS2, installation of steel-flex lines in the wheel circuit (between pressure modulator and brake caliper) affects ABS regulation, which modulates pressure based on change of volume in wheel circuit. That is, due to a different volume-to-pressure characteristic of the lines, the same change in volume will correspond to a different brake pressure. This changes the control levels of the ABS control.

BMW does not approve of installation of steel-flex lines in wheel circuit.

Hint: Installation of steel-flex brake line between hand controls and ABS Modulator can cause a change in the brake feel; however, BMW sees no safety-related limitations with regard to the residual brake function.


Should you discuss this matter with your customer, advise him about the issues described in this service bulletin. The installation of steel-flex brake lines between ABS Modulator and the caliper is not permitted by BMW.
This also pertains to steel-flex brake lines from third-party vendors.