Hydraulic Unit

WARNING: This page does not describe the selected car, but rather 21 other vehicles, including the 2003 BMW M5, 2003 BMW 540i, 2003 BMW 530i, 2003 BMW 525i, and 2002 BMW M5. However, it is still accessible from the selected car via links, so may be relevant.

NOTE: Hydraulic units are model specific and not interchangeable.

To control braking system pressure, brake hydraulic unit has a 3-way valve which permits 3 separate brake pressure conditions. Pressure build-up, pressure retention and pressure drop are the 3 pressure phases that adapt to vehicle braking requirements. Control procedures are as follows:

As soon as wheel lock-up is indicated, brake fluid pressure is reduced. If wheel still tends to lock, pressure will be further reduced until wheel acceleration or slip limit is detected. Pressure is then raised and the control phases repeated. These pressure changes occur in milliseconds.
The return delivery pump returns brake fluid from wheel brake cylinder, while dropping pressure to appropriate brake circuit. The pump is a 2-piston pump design so circuits of a dual brake circuit system remain separated.