If the car has some miles on it, just replace the master cylinder for safety reasons and try gravity bleeding all 4 brake lines. Then try pressure bleeding again. If anything you may have to bring it to Mercedes or an independent shop with a scan tool that bleeds the ABS module as well

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2001 E55

2001 E320 Sedan

2001 Subaru Legacy

2006 SLK280

2016 Subaru Outback 3.6