I usually remove the calipers carefully, and pre-rinse with clear water in a pan...don't want that going into our storm drains. Use a shoe lace to clean the exposed pistons so when you push them back into the bores they are clean going in.
Bleeding rear brakes can be tricky because master cylinder is not the "highest" point, so air bubbles can be stubborn, do not be afraid to have help pressing the pedal while timing the opening and closing of the bleeder valve and tap and wiggle lines to dislodge air bubbles.