As an experienced mechanic, I will never be more humbled than I was by this simple solution to a problem that caused me many a wasted evening.
OK. New clutch master and slave cylinders. Install. (This is the crux, but read on.) Add fluid. Crack slave bleeder, attach tube, put in bottle. Pump clutch pedal ten times. Look in bottle. Nothing. Repeat ad nauseum. Zippo.
Tried gravity bleeding. Kinda, but not really. Zippo.
Get out Mity-vac. Set it up. see a little fluid. Repeat, ad nauseum. Zippo. Open beer and give up.
Finally, online, I found this:
"I think the biggest reason it is so hard to bleed the clutch system is because the bleeder valve is not higher than the incoming fluid line. Gravity bleeding will work to clear the clutch line of air, but there will be a bubble left in the slave cylinder. What I found that works is to simply unbolt the slave cylinder and hold it such in a way that the bleeder valve is at the highest point, relative to the incoming line. Holding it this way, the air bubble in the cylinder will naturally float up and out by gravity, e.g. the slave will fill up from the bottom and push the air out the top. Since gravity is doing all the work, this is a one man job. When the air bubbles stop, close the bleeder and attach the slave to the bell housing."
Slapped myself in the forehead and caught a lesson in humility.