I experienced very similar troubles when bleeding the brakes on my car.

We finally determined that we think that the air was being sucked in around the threads of the bleeder valves, and mixed into the fluid stream coming out of the bleeders into our clear hoses. So yes, coating the threads with teflon, or buying new self-bleeders that have a similar substance coated on the threads might be a good solution.

Also, you might want to check your rear axle bleeder seals. We removed the rear bleeders and there there was crud at the seat of the bleeder inside the cylinder. That crud may cause an improper seal on the rear end.

We ended up bleeding 2-3 whole quarts of brake fluid and still had intermittant air issues that we couldn't explain. But, after we gave up and started the car and drove it, the brakes worked beautifully.