Bench bleeding new master cylinder

Had to replace both rear brake calipers this weekend. Then I noticed that the master cylinder was leaking. Might explain why I was losing fluid occasionally. I bench bled the cylinder and bled each caliper, RR, LF, LR, and RF. BUT, I can't seem to get all the air out. So, I decided to bleed the master cylinder again while installed. Again, I can't remove all of the air from the calipers. I see a small amount of air bubbles. The car brakes fine but the air bubbles are going to bother me. Keep in mind that it took almost 2 hours for the brakes to finally become firm.

Maybe I'm drawing in air somewhere? Any tips on getting all the air out?