The hole that the fluid passes through to go from the reservoir into the master cylinder is pretty small, and can get clogged with crud. If the hole is clogged, no fresh fluid can get through. If you have acces to an air compressor you can try blowing air through the system backwards (from the wheels). That's what worked on the Plymouth. If not, you could try removing the reservoir from the master and cleaning it out manually.

By the way, thanks for the welcome! I've been lurking for a while, but haven't had a reason to post yet.