You need to remove the front
Brake Caliper and the brake caliper piston, remove the caliper piston seal, discard the boot seal, remove and discard the piston seal, remove the bleeder valve and cap, clean the brake caliper piston bore and seal counterbore, and the caliper piston, dry the caliper piston bore and counterbore, and the piston, inspect the caliper bore, and inspect the caliper piston.