You'll have to bleed the brake lines.
After all this time it wouldn't hurt to get rid of the old brake fluid and replace it with fresh fluid. If you have taken it apart and the cylinder looks "good" you could clean it and use a rebuild kit (if they are still available) to rebuld it. (ir replace the seals.
Or you could replace it with a new shiny one, new pads and don't skimp on either having the rotors turned or replaced.
Then rinse and repat for the passenger side
Yes, it's a project
