AOD cooling line leak

[calpolymustang]

I put an AOD in my '66 Fastback and have a leak at the cooling line fitting. I am not familiar with this type of connection, and have no idea what the black plastic part does.

I thought the lower line was the radiator return line.

Any help is appreciated.

 

[HISSIN50]

Is the black stuff actually plastic or thread sealant?

 

[calpolymustang]

Its some sort of plastic fitting.

Its not RTV or teflon tape.