Bubbles in my master cylinder

[Stout72]

After discovering that contamination virtually destroyed my brake system, I've replaced nearly every major component except for the plumbing. So far I've been unable to bleed all of the air out of the rear brake system. After going through about half a gallon of brake fluid during the bleeding process, I happened to notice that a stream of tiny bubbles would rise from the forward port in the bottom of the master cylinder reservior for the rear brakes each time my assistant slowly depressed the brake pedal. This is a brand new Bendix unit that I carefully (so I thought) bench bled on the truck.

Thinking that either the new master cylinder was bad or I screwed up in the bench bleeding process, I returned the master cylinder for a replacement... another new Bendix. This time I carefully bench bled it, while it was clamped in a vise, according to the mfg instructions. Same result. So I must be doing something wrong.

Suggestions or words of wisdom greatly appreciated. Otherwise, It's going to the brake doctor.