My '06 749S is doing this same thing! Just contacted my dealer and they are going to warranty the master cylinder. They have bled mine twice already but I don't know the mileage it actually loses pressure because I don't use my rear brake much. Take it to your dealer and have them fix it gratus.mark_mtb said:My 749 rear brakes loses pressure every 4000M sometimes earlier...I usually just bleed it and its fine till the next 4000M or whenever it decides to lose pressure again. Is this normal for these bikes?