Sounds like the cable may have rusted up on you. Do to the lack of use. Its not always easy to release it when this happens. I would first block the front tires so that it can not roll over on you. Then go under and check out the adjustment were both lines meet from each back wheel. Spray something like WD-40 over the full cable & pull down on the adjustment part. If they are tight. Then you may need someone else on top to pull on the Break release . If you do not feel anything when they do this. Then there maybe a problem up on top were the peddle is. But most likely, it will be in the back two cables. This is were mine always gave me problems at. I had to put new cables on the back, because mine had to much rust in them. You may get lucky & after a good coating or two with that WD-40 or rust remover of some kind. They may just release for you. If so, make sure you keep them coated. Also do the same on a new set if you need to buy them like I did. Plus use them more so that the rust can't build up inside them again......If you do have to buy a new set. You will have to raise the truck up in back. Take off the wheels & drums to replace the new cables. And it is a dirty job.... .....Good Luck ..............M.C.S.

As far as I can remember the park brake is located on the rear of the trans, or with air brakes it is your service brakes that are also the park brakes. So does your truck have air brakes or hydraulic brakes. Once that is knowna better answer can be given.