Had a similiar issue with a buddies car that he bought out of Lousiana. The clutch pedal was giving no pressure. Put fluid in, no leaks. Checked for air in the lines, no air. Did the same as you, I replaced the slave and clutch master cylinder. I ended up looking over all the lines. Turned out to be a small pinhole in the hose that runs nearest to the battery. Gave no signs of leaking but I did notice a tad bit of rust. When I ran pressure from the slave back into the system it pushed out in the rusted area. Just something to think about.
Do an inspection on the lines as well as connection points and keep us updated. BTW, have you considered going to a 1pc SS line in the future?