First, do you know if the master cylinder ever went dry during the job? Second, you may want to try gravity bleeding, where you simply leave the bleeder screw open and let gravity rough bleed the brakes. I think it takes no more than a few minutes.
There is a possibility that the rear brake hose has internally disintegrated... but I don't see how that could affect both rear brakes.
Beyond that, I too remember reading about losing half the brakes when something goes wrong with the master cylinder internals.
95 E320 Cabriolet, 131K