If penetrating oil doesn't loosen it, then a cable replacement is in order. In most cases, people don't bother using the parking brake. I'm among that category. However, I did end up replacing the parking brake cable due to lack of use over the years.
On my 79 El Cam it was the front cable that was bad. Really stuck. Had to remove the pedal assembly. First remove the little wire from the release handle it is very fragile. There is a switch at the top you will need to un plug a connector from. (Turns on the brake indicator light on the dash.)
There are 3 bolts that go into the body one at the center top and one in each lower corner. Then I had to get it away from the body took a little fighting to get it out.
I used a huge pair of channel locks from a round hole in the gear part with the teeth to the side of the assembly to get the cable to move enough to get it removed from the clevis.
Mine had a wire clip on the top of the cable housing where it mounts in the pedal assembly. Later ones have spring fingers holding it in.
The GM replacement cable from Rock Auto was for the later cars, it mounts differently (spring fingers) but works fine. NAPA had one that was the same as the original. Both replacement parts have a spring on the under car part of the cable that the original did not have. Had to get one of these to make it work. #5 BRAKE CABLE SPRING RETAINER, FRONT PARK BRAKE CABLE, ROUND Part #850780
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