Member, Sport: Off DSC: On (>50 posts)
Join Date: Mar 2006
Location: Cave Creek, AZ
Thanked 13 Times in 11 Posts
Have you replaced all of your brake hoses on the car, including the 2 under the hood on the passenger side of the engine near the abs pump?
I am not a professional mechanic, but to my knowledge, the major sources of pedal sink are: hydraulic leaks - either external or internal, expanding rubber hoses, or air in the system. I use a power bleeder that pressurizes the reservoir on the master cylinder while adding fluid and have had good results with that method. I use ATE SL6, its a low viscosity fluid that is BMW backwards compatible, here is the ATE SL6 description (I do not track my car) hope this helps:
"The Ate SL6 fluid is a low viscosity formula for modern ABS and DSC systems. BMW recommends low viscosity brake fluid for all chassis beginning with the E60 5-series. Low viscosity (LV) is a thinner fluid that works better than a normal fluid in the small channels and passageways in the ABS/DSC pump. Using the heavier fluid will result in a delayed response to ABS and DSC activation and a spongy feel in the brake pedal (only during ABS stops). Ate SL.6 falls just short of DOT5 specs so is classified as a DOT4. It can be mixed with other DOT4 or DOT5.1 fluid. For track use, a heavier high performance brake fluid such as Ate Typ 200 or Motul RBF600 or RBF660 is recommended because of their higher boiling point and because the thicker fluid will thin out at track temps. But for street use we recommend switching back to a LV fluid, especially in colder climates."