passenger rear wheel locked

get both wheels off the ground or it will not turn with PT. Pull the suspected axle if it is still frozen with drum off. Pretty easy to do with 8.75 and inspect the axle for bad bearing.

On the master cylinder different issue likely, look under the dash where the master cylinder push rod goes into the master it is likely wet there. Or are you saying it rolls fine without brake applied? and when applied with the situation described it locks on that wheel?