If you wait until they're locked up, then loosen the mounting bolts on the master (so the pushrod gap goes huge) and the brakes are still tight, I would look at the soft lines.
For that you can crack a bleeder at the wheel to see if that wheel suddenly frees up (or not).
I have a feeling it's the flex lines, they're the only part I haven't directly tested yet - I'll give this a try tomorrow. The pushrod gap has grown a comparative great deal since when I started chasing this problem and the light duty brake behavior hasn't changed hardly at all.
