Three-step check for basic forks
1. Assume the damper rod retaining bolt is corroded in place. Apply generous amounts of penetrating oil or heat if required before attempting to crack the seal. A snapped bolt is a whole lot harder to remove.
2. Inspect the plastic piston ring on the bottom of the damper rod. They can go brittle with age, crack and cause debris to slosh about inside the fork. If it looks to have seen better days, replace.
3. Have a good look at the top plug. It may have no longer been sitting in oil, and can corrode as a result, like this one from the TS. Dress with emery paper to remove any damage or high-spots and replace the seal. Job’s a good ’un.