Sounds like your on the right path, but I wouldn't worry about the master brake cylinder yet!

Also, when was the last time the brake fluid was changed? Should be changed every year, or two. Look at the brake fluid reservoir and see what the color is. If it is clear it's been changed recently, but if it's yellow, or amber, or darker, it's old. My guess is that it's old and has moisture in it which has corroded your rear brake cylinder causing it to stay extended thus wearing pad.

Let us know what you find.................BB