Are the posts breaking on your old cedar fence? Yes, sir! They rot off right at ground level. Now you have the job of digging around it, pulling out the old concrete, and setting a new post. That can be hard work getting the old concrete caisson out. Perhaps there is an easier and neater way.
In most cases, old rotted cedar posts break off right at ground level and are punky or mushy below that level. If this is the case for you, just simply scoop the old debris out of the caisson. Once it is scooped out to a depth of 16" or more, tamp down the rest of the debris at the bottom of the hole. Since your new post may be a little wider than the hole in the caisson you may need to shave two adjacent sides of the new post. Save the pieces shaved off as these will be used to tighten the post.
Now take the shaved end of your new post and insert it into the old caisson and tap it down so it goes to the bottom of the hole. Using a level plumb the post. Once the post is plumb you can re-attach it to your fence rails. Take the wedges you shaved off from the post and tap them down along side of the post making it tight in the hole.
Now go buy yourself a Starbucks.