Why did you bleed the brakes at all?. With all types of brakes one never has to diconnect any of the brake lines. The only reason could be a defective master cylinder or one of the pistons in the individual calipers and other hydraulic gadgets (ABS, ASC + T etc.). If you repair / replace any of these air would enter the system. Only other reason is to change the brake fluid. BMW recommends a yearly change. In a well functioning break system air does not accumulate. How should it. Regards Ekkehart