A couple of ideas... (don't be insulted, I don't know what your level of expertise is)... Did you pump up the brakes to seat the pads? This should only take a few strokes. After all was done and everything was buttoned back up, did you check for fluid leakage after testing the brakes? Since you checked the front brakes after finishing them, your problem should be confined to the rear brakes.
You either have fluid leakage from an open source, or you still have air in the lines.
95 E320 Cabriolet, 131K