Brakes to soft after conversion

"Did it ever work" correctly (after the conversion?) If not the master may be too large bore. You shoes or even pads could be glazed. The shoes may be effectively too small if the rear drums are worn. You need to have them arced. Good luck finding someone who has a machine, anymore. Regardless, though, you should be able to get one end to slide. How is the pedal feel?

AND IS IT power or manual, and which was it origininally?