I understand most later day ABS brake systems require a vacuum bleed at the caliper or the common brake-pumping bleed. Pressure bleeding at the master cylinder is not recommended. I suggest you try vacuum bleeding at each caliper and draw fluid from that direction. Doing so could reposition internal M.C. valves to correct your problem. I don't know for certain, but it is surely worth a try.
The best vacuum bleeding system I know of is the pneumatic kind that uses an air compressor. I own
the pneumatic one on sale here for $31.99, a consumer grade vacuum pump from Harbor Freight. The ones at our
church CARS ministry are commercial grade versions of the same thing. They offer a friendly one man operation for vacuum bleeding of brake lines.
Good luck!
