How is air getting into my lines?

1967 Dodge Dart 270 /6

I just swapped out the master cylinder and the wheel cylinders for the front brake drums (9"). I also installed new hardware, self-adjusting screw assemblies, and new shoes for the front brake drums. I proceeded to bleed the brakes, which went fine for the rear brakes and the front passenger brake. However, the front driver's-side brake has given me a little problem.

When I bleed the brake, a bunch of air jets out of the bleeder, followed by fluid. I bleed it correctly and get it squared away, but the pedal feels squishy. So, I have my buddy pump the pedal until it feels firm, then have him press and hold it down. I open the bleeder, and again, a bunch of air shoots out, followed by brake fluid.

This happened five or six times before I gave up. I don't want to drive the car until I can figure out what's going on.

I can't find any leaks (in that there's no brake fluid dripping from the car), but the hoses and lines are pretty old and crusty. The connections are all tight. The only part I can't test or inspect easily is the little box that distributes brake fluid to the wheel cylinders[EDIT: Just found out this is the "proportioning valve"].

Thoughts?