There is a liquid radiator stop leak. I do not reccomend using any type of stop leak chemicals. They are temporary fixes and usually for smaller leaks. They can cause problems by clogging coolant jackets in the engine block and cylinder head. They can also cause blockage in your radiator.
I had a customer that had a radiator leak and he used some type of stop leak chemical that clogged his coolant jackets, and it blew a head gasket shortly after. You can replace the radiator or risk more expensive repairs down the road.
I guess the days of cracking a raw egg into the radiator are over ! So I'll go the replacement route and continue to monitor the coolant level in the left side reservoir. Should I add coolant there or into the central opening (after the engine cools, of course)...?
Always add coolant to the radiator. The overflow tank can be full and your radiator still be low. Monitoring the coolant level in the radiator is also way more important than monitoring the overflow tank.
Thanks, Peter. I think I just saved some bucks here...!