You could try warming them a little from the outside - expand the y piece a little - and working some WD40 into the join. Brace the headers with some timber (take the weight/impact load off the fittings at the cylinder) and light rubber mallet action on each side of the y-piece to loosen it off.
If it is really b*ggered you could always cut the Y-piece behind the headers (careful not to cut the headers) and use an extractor to pull off the remaining sections.