heater core leak
 

My 1994 Century developed a small leak in the heater core. A few drops on the carpeting, nothing huge. My mechanic quotes of $1200 replacement job.
What the heck!! Is he kidding?
Anyhow, I bought a jar of Bar's stop leak additive. Put it in, and it seems for now to have plugged up the leak. Is this a good fix? If necessary, if leak gets worse, can I do the replacement? Is it a difficult job?
What do you people out there with a similar problem think? Ed