Thanks everyone! The leak was sporadic, but I did not make the association with using the San-T-Flush. That, in fact, is when the leak occurs (confirmed today). The leak is because the plastic backflow valve connected to the brass valve is broken where it attaches to the red water line. Now, thanks to all, I know that I can temporarily fix the problem by capping the 2 water lines until I get the replacement valve. Thanks again.
Don