Is it the master that sticks out 90* from the firewall
or the one that sits at about 45*
there could be air trapped in the masters or a line that refuses to come out.
I have a master from an 88 Chevy pick-up and the slave from an 86/7.
90* master----The only way I can get it to bleed, is to jack up the rear of the car. Seems a bubble gets hung up in or near the master and needs to purge thru the resevoir