You won't remove anything gunked up with WD40, use plusgas.
Re check to make sure the piston in the cylinder is not being stopped from comming out, didden circlips are common.
Remove the banjo, seal the banjo thread hole with a bolt, making sure it is sealed and use the right thread. With fluid in the reservoir, slowly pump the lever, The fluid can't push down the hole for the banjo so should pressure push out the piston - assuming you have removed all the fittings.
If you cant find a bolt for the banjo, reconnect the brake hose and put a wooden block between the pistons on the caliper so the pistons in the caliper don't pop out.