Radiator leak

[Forker]

I just went out and filled my radiator to the top and seem to have a leak once I started it up. I can't determine where the leak is coming from, but I have antifreeze leaking when it runs. I filled it with distilled water btw, but the previous owner must have filled it with antifreeze.

First question I have is can you over fill them and if so is there an over flow function that rejects too much water.

Second question is would it be a mistake to add radiator sealant to stop the leak?

Thanks, I was hoping to use it this weekend for a nice drive.
george