[wfieldin]

Are all calipers mounted so that the bleeder is at the top of the caliper, not below the inlet (brake) line. You could try and do some isolation, disconnect rear brake lines and plug at the junction on the rear end, bleed front, then hook one rear back up, bleed, and then the last. Seems tediuos but may show where there is a leak, I would have to say it is just air in the lines. The bleeder screws that are check valve may also help, but in the end my problem was just air trapped in the caliper.