Several have already suggested to remove the residual valve under the brass seat in the rear brake port of the master cylinder. Here's how to do it.

Using a wood screw, screw into brass seat just a few turns. Then use a claw hammer or wire cutters to grab hold of the screw and leverage the brass seat out of the master.

Remove the residual valve and spring, and reinstall the brass seat.

This should resolve the rear brake drag assuming the brake pedal linkage is properly adjusted.