Just a quick thought:
On a Mercedes in the mid 60ies, they also had the front disk, rear drum arrangement.
HOWEVER: Calipers and brake drum wheel cylinders I believe work on different tensions/pressure, and different release current (??).
There is a valve that regulates the rear pressure in the system, located (on the Mercedes's) under the rear center of the car before the lines splice to right or left.
I would bet something similar is built into the BMW system and this young Mechanic of yours, may have not have removed this, therefore the ...........
Just a wild guess.
It would be nice to hear back , right here, if that or any other cause was found and the issue was resolved !!
Last edited by 6point3; 05-21-2012 at 02:25 AM.
Reason: not mid 20ies, but mid 60ies....lol