I'm new at this too and just posted a similar situation. My pipe too is rusted and the water level is going down. I was told by the head of the "Safe Dams Dept." in Georgia that the old pipe underneath my dam needs to be filled with concrete after I install the siphon drain.
Now I just need info on how to make that permanent siphon drain.
It would take a lot of concrete to fill the pipe you have though.