Maybe try a different shop. I would say that there is a good chance that the problem was the master cylinder and they just aren't getting it bled. There is no way the booster can be at fault for the rear brakes not working so that says something about the shop. That being said, how do you know the rear brakes are not working?

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)