I had similar problems bleeding the rear breaks on my 240D a few weeks ago. I found out that the break fluid reservoir had two chambers, and the one that supplied the rear was empty. I had to fill the main res. to almost overflowing for the rear res to get any fluid. So I had to nearly overfill the main res several times to fill up the rear res. You might look carefully at the res. on top of the booster to see if you can see if it has two chambers.

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