most likely cause of your problem is a piston that's stuck in the bore due to corrosion. The small bleeding hole is blocked by a mixture of rust, corroded aluminium and gooey old brake fluid.
Only cure is to completely dismantle the caliper, thoroughly clean all parts and inspect for irrepairable damage such as deep corrosion marks in the sealing surfaces of piston and bore. If all cleans up well, assemble with new seals and your problem is gone.
But do not work on your brakes if you don't know what you're doing!
Cheers
Andy
