I have a 25 year old garage with a cinder-block chimney that is leaking. I really don't like the looks of using tar, but will use it if that is the only thing that will work

Does the leak show up at the ceiling or the floor only?

Ceiling might be the flashing.

Floor only might be the cap.

Let me know,

KBC

Cinder block should not be used as a chimney for this very reason. Do you mean water is leaking in or is smoke and creosote leaking out of the chimney?