No, you don't have to remove the exhaust... The rear brake caliper simply unbolts with 2 bolts, then lifts of the caliper.I am working on the brakes and thank I may have to rebuild the rear MC. The book I have says to remove the R exhaust to get it off...
Edit: Sorry, I thought you were refering to the rear brake Caliper.
Edit: Yes, the rear Master Cylinder is a much more involved job, involving removal of the exhaust system.