brakes wont work

[fredscarryall]

i have a 1950 suburban with an aftermarket firewall mount power booster, 1970 chev truck master cylinder,1978 ford granda new front disc brake setup the came with my ifs, used 1994 lincoln mark 8 rear with discs brake set up (disc where resurfaced and new pads),and a new aftermaket disc/disc portion valve mounted on the frame. i have gone through my forth master cylinder and the brakes wont bleed and i cant get a pedal. the last master cylinder bench bled perfectly on the bench but as soon as i get the lines hooked up it doesnt bleed. i have power bled, open up the bleeder and tried to let it bleed on it own, and ran a tube from the bleeder to a bottle filled halfway with fluid and pumped the pedal. nothing happens. what could be the problem? i think we have spent about 6 hour trying everthing. there are no leaks anywhere. anybody ever have this problem.