I had the same brake issue and let me add a few comments.
. Brake booster test on youtube are inconclusive mine passed
. Experience told me I had a vacume pump,line or booster failure
. The easiest thing for me was to put a vacume gauge on line at booster read 25 inches knew I had booster failure.
. I also ordered mine from salvage yard be mindful between dis assembly at yard and shipping things can get out of order. On mine found master cylinder rod in wrong. Comparing new to old with a detail eye.
. I presure bled mine but still had poor peddle. I took it to a dirt road and activated the abs about 4 times in the dirt.
. I bled mine the old two man way with the engine off and it is now great
. Thanks for the great step by step

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Astrok
2005 X5 4.8IS
2008 X5 4.8
2005 X3 3.0
2006 MB R500
2001 Slk 232