Hi Dave, I have had both the Master Cylinder and Rear Caliper apart. What I've found doing this is,
The Master Cyl. cup will have failed from dirt and heat. You'll find grooves and scratches in the bore and on the piston. You must get them ALL OUT, ON BOTH PARTS, then Polish them back to a good shine. Replace the cup, and the rebuild will last some time.
On the Caliper, You'll find the Stainless Steal caliper pistons showing signs of corrosion. Water, dirt, heat and the Alum. bore causes this. I just cleaned and buffed the pistons off and cleaned the bores up, put it back together REAL CLEAN and it's working fine.