i had exactly the same issues with my 66. it turns out its the slave cylinder sucking air, not the master. the seal around the main shaft of the slave, it will suck in air if worked fast but when done slow it wont, and at times will even bleed the air it sucked in back out. if youll look, the bleeder is at the bottom, thats because the air works it way that way naturally. if you were sucking air at the top, thru the master, then you wouldnt be able to bleed the spongy feeling out in any thing resembling a short amount of time, it would need to be physically bled to get rid of it. you mentioned you have changed the master a few times, when was the last time you rebuilt the slave??

i must disagree with terry in one point, i put a rebuilt master and slave in my truck, both came from autozone or oriely i dont remember(the napa here wont get my bussiness). both were installed on the same day and i flushed and bled the pipe connecting them and used quality fluid and a good bleed on the system afterwards. i drove the crap out of that truck for 5 years myself, then the following owner did for 4 years afterwards... last i heard they were still going strong. i dont exactly agree with the 3 year limit. if you rebuild your system right, keep up on maintanance, then it ought to last quite a while. i do agree with the stainless sleeves, definatly better then just reaming the casting. dont let it sit around and gather dust, keep clean quality fluid in it, and dont mix an old dirty part with a fresh new part and you can rely on many many years of flawless service.