Carl

You have a leaky rear brake cylinder. When air gets in there, it's nearly impossible to bleed the brakes- the pedal just goes to the floor. But the trick I used was to open the bleeder valve in the rear cylinder, then pump the brakes rapidly until fluid squirts all over the back, then bleed as normal. The pedal pressure should come right up. Then don't let the fluid level get low, or replace the rear caliper unit, either way.