Member, P500, DSC On (>600)
Join Date: Oct 2009
Location: The Netherlands
Thanked 102 Times in 80 Posts
My bet is on water in the booster too. Had this happen on an old style VW Passat, blocked drains. Booster had about 0.25L water in it, which would freeze at below zero temps, causing a rock hard pedal. Only thing to do about it is to replace the booster as the water attacks the metal housing, plate and spring inside) and include cleaning that drain plug mentioned above in your regular maintenance. I believe it's mentioned in the TIS too.
1998 E39 M5 (yes, that's right, going EXTREMELY strong and ZERO oil/rattle issues), Carbon Black/Caramel Full/Alcantara.
1985 E28 525i, Alpine White/Pacific, 12 yrs of ownership, DD during winter, most reliable car ever.