When I bought my '90 used, it had a very persistant bubble that would not bleed out. Finally, I took the hose, caliper and master cylinder off the bike and hung it up on the wall with the caliper up. I let it hang overnight, and tapped the brake line with a screwdriver handle from time to time to knock any bubbles loose.
Next day, laid the hose on the ground with the caliper elevated, and bled it off the bike. No more bubble! It was a PITA, but I wish I hadn't wasted so much time and fluid bleeding it the normal (and ineffective) way.