Started the day with the idea that I should do a simple brake fluid change before the weather turned to chilly, if it ever does this year. Anyway, the plan was to pull about 80mL of fluid through the passenger rear bleed valve with a mity vac hook-up. About 40 mL came through then bubbles then just air. The volume in the reservoir didn't decrease any visually. Tried for at least 30 minutes and just got air with no change in the reservoir. Then tried the driver rear and had the same thing happen. However, if I bleed the front calipers fluid come through and the reservoir goes down.

What did I do wrong and how can I fix it?