OK, here is what we have learned:
There is not a backflow valve
If the tank is full, the water backs up at base of toilet.
The 1/2 bath toilet and sink, as well as the sink in kitchen are all on one system.
The indicator lights ( ones that let you know how full the tanks are ) do not always work.
So, light said tanks were at low level, but back tank was, in fact, full. So, it leaked at base of toilet. From now on, we will empty tanks on a regular basis and not go by the indicator lights!!!
I do appreciate both of you guys replying to quickly!! And, I love this site!! We will be joining the club soon!!