how to remove brass part from steel tank?

I bet its knurled and/or soldered in place. More heat, more force. If its threaded, you'll still need more heat and force!

if it is a pipe thread , have you tried tightening it a little before attempting to loosen?

old pipe fitters trick

Good Idea, but it won't float. The ammonia will dissolve the zinc out of the brass and just leave a brittle mess. Any pressure will break it.

From the description and header I assumed that was the idea, to get rid of the brass part. Don Young