coolant leak - by radiator drain plug

I did a complete coolant overhaul with the kit from Tisher
but still having a leak. The leak is by radiator drain plug.

The location similar to this video:


I've contacted the guy who posted the video and he said
it's the o-ring between the expansion tank bracket and the
radiator. Mine was intact and but I replaced it.

How tight is the fitting between the bracket and the radiator?
Mine is snug but I can pull it off without a lot of force.

Anyone have any suggestion? I'm looking into getting a new
bracket and replace it but can the plastic piece on the radiator
side be the culprit also?

Thanks in advance.

I am having the same problem and will take off the expansion tomorrow. I think its the o ring. I will find out and post my results. Good luck