Here is an easy fix. Put in some GM Coolant Seal Tabs. You can get then from the Chevy dealer. 5 tabs in a package for around $5. I had a leak behind my water pump in my Dodge 318 one time. My mechanic said to 'Put a couple of these in'. I had nothing to lose, so I did. Dropped them in the radiator and drove it for awhile. Stopped the leak. That was about 4 or 5 years ago. Damn well worth a try. Good luck.