Is the pedal spongy, or does it just fall to the floor?

Two questions on your rear brake installation:

1. When installing, did you properly set the star adjusters per the service manual?

2. If you did not do this prior to bleeding the brakes, have you removed the rear drums and made sure the cups and actuators for the rear wheel cylinders are still in their bores, and the star wheel adjusters still in place on the correct sides?

And although the fronts had no issues before, it might be a good idea to pull the drums and take a look and see if there is any seepage from the front wheel cylinders, since it appears you may have issues with this circuit now.