Usually the handbrake mechanism seizes up and the return springs are no longer able to fully return the mechanism to the fully off position. Eventually the mechanism stays fully on and does not move at all when you pull on the handbrake.
Sometimes you can free them off with releasing oil and working them by hand, also some fords used a similar caliper with a stronger return spring, these can also help. Usually a new or recon caliper is needed. Worth checking the cable is not seized before you buy calipers!
