Yet Another Bleeding Problem...

If the above tips don't help. Try this.
Remove the lines on the master and plug off the ports. (Use steel inverted flare plugs, available at any GOOD auto parts store). Pump the pedal a few times and it should go rock solid. If it doesn't, bad master.
If it does go solid, hook up the front brake line. Test the pedal, (you may have to bleed the system again). If you get a good pedal, the problem is probably with the rear brakes.
(Obviously).
Either way, it narrows down where the problem is.
You could go farther and cap off the hardlines where they connect to the flex hoses to see if there is a possible problem with the hardlines themselves. (Pinholes or whatnot).