I've done one of these. There is a sealant around the top and bottom of the brass tank. I used kerosene to soften it up and scrape it out over several days.
If you managed to save the four rods and nuts- put them back in but install the nuts so you will actually use the four rods as spreaders to seperate the top and bottom castings from the brass tank. Go slow tighten the nuts just a little each day, tap with a rubber mallet, keep scraping out the sealant. The two pipes on each side of the tank are just fitted in the grooves and aren't threaded.