OK, I was able to loosen the chewed nut with my big vice grips... as soon as it broke loose, fluid started running out of it. So I tightened it back up. Then I loosened the end of the flexible line from the wheel side ... fluid started running from there too. So this seems to indicate that there must be something in the braking system preventing the fluid from getting to the bleeder valve. I'm new to these braking systems, so how much of a job is it to get in there and find out? Any ideas or likely culprits? Longbox55, you mentioned the wheel cylinder.

http://captaincheeto.com/pictures/64-gmc/brakes/IMG_8847_1024x768.jpg

There is one place that you have not looked, and it is there - only there - that you shall find the master.