9" rears dragging

CHECK THAT THE MASTER CYLINDER IS FULLY RETURNING

In the bottom of the cylinder reservoir(s) are the return ports. You should be able to press down the pedal (have someone) and watch as it returns, you should get a spurt of fluid up through as it releases

Another way to check is to duplicate the problem, then crack the master line(s) and see if there is any remaining pressure, and if the brakes release