You will need to actually get your hands on the brake caliper assembly to resolve this issue.
Note that the linkage itself may be free but the actual brake puck (pad) is probably stuck to the brake disc.
Generally all you have to do in a case like this is to spray the brake caliper assembly (make sure you spray small pins next to the mounting bolts) with your favorite brand of penetrating "rust bust" product. Wait a few minutes for the penetrant to work, then tap on the caliper assembly and disc lightly with a small hammer.
Do the tapping with the parking brake in the OFF or released position. Make sure you have a wheel chocked or have jack stands under the tractor to guard against rolling when the brake frees up.