Usually with air in the system, there is a distinct feeling like you are compressing a spring. Is the rear brake setup like the old twinplex system with one shoe on top and one on the bottom? In that system, the wheel cylinders are vertical and the bleeder is in the middle. I believe that you have to back off the adjusters on the top shoe before bleeding the cylinders.
If you didn't have the shoes adjusted out properly after, then it may be causing your problem.
