Betty has been good for a while! So don't get sore at her. She's old

Here are the facts:

New rear wheel cylinders and shoes.
Betty stops straight as an arrow until I apply hard pressure to the brake pedal and she REALLY tries to bolt to the left.
Here is what I HOPE is wrong: I did not do a good job bleeding my new right rear wheel cylinder. I will bleed it again this weekend and hope for a cure.
What I THINK is wrong: One or more of my rubber brake lines on the right is/are closing up under heavy loads.
As you all have told me time and again...check the simple stuff first. I HOPE it is some air in the system. I don't want to fiddle with the rubber lines.
What are your thoughts? Am I missing something? LMB RULES!!!!
