Bleed both sides of the rear brake circuit. My opinion is if you're replacing one side of the brakes, they should both be done as both have been in service for the same amount of time. If the brake pedal wants to bounce back at you and/or feels spongy, it's an indication that air is still present in the system. I would completely change the brake fluid in the system (siphon the old fluid out of the reservoir before filling and bleeding to save time). Check the handbrake adjustment as well.