BMW E28 brake pedal sinks to the floor
Why does the E28 brake pedal slowly sink to the floor with the engine running?
The E28 has a hydraulic brake booster, not a vacuum booster. The most common cause is a faulty pressure accumulator, or brake bomb, whose diaphragm leaks so the booster gets no pressure. The symptom is a brake that softens and a pedal that sinks to the floor after a delay. A leaking pressure sensor can give the same symptom.
Where are the pressure accumulator and pressure sensors, and which should I try first?
The pressure accumulator is behind the hydraulic reservoir, in the right corner as seen from the front of the car. The pressure sensors are on top of it and in the valve block on its side. Replacing a sensor is cheaper than replacing the accumulator, so try it first. Leave the brake work to a workshop if needed.
Can the pedal sink to the floor even if no brake fluid is lost?
Yes. If a brake line is broken, the pedal can sink almost to the floor and the car pulls to one side even though no fluid seems to be missing. The rear brakes have their own lines, and the fault may be on one side only. Check the lines and connections before replacing larger parts.