I guess you’re trying to pump the air out. Just loosening the nipple will allow air in the cylinder to escape as it’s always under slight pressure from the reservoir. You might need to twist and turn the caliper slightly while tapping it with a screwdriver handle to help dislodge any small bubbles stuck inside.
The rear brake system is a bit of a pain to bleed because bubbles can also get caught in the master cylinder, slacken the banjo at the top to get rid of any air trapped there.