ABS systems can be a PITA to bleed properly. I have an Expedition with ABS, and the factory recommended bleeding procedure involves hooking up a $2,000 piece of equipment called a NGS Star Tester to work the ABS solenoids while bleeding in order to evacuate air from around the solenoids. I wonder if BMW employs a similar method.
BTW, is DOT 4 recommended for this vehicle?
Also, if the brake hoses are original, now is a good time to replace them.