I had this SAME exact problem on an '88 toyota truck several years ago...if my memory serves me right the master cylinder was bad. I remember thinking it was odd too...because it didn't exibit the typical bad master cylinder symptoms. Even with the new one installed I had a Hell of a time getting it bled. I ended up have to use a pressurized brake bleeder to force fluid through the system via the resevoir. That rear axle proportioning valve was a pain to deal with during the bleeding. Good luck.

2007 Ford F150 Supercab