Mate it may sound odd 
but wet them up with the WD 
^ You could do the same and heat it. The expansion and contraction can also loosen it.
s
Yes heat is a wonderful tool 
 and heat and quenching  can loosen up things that seen impossible to move 
One must be mindful of the materials being worked on 
the repercussions can  give a result that is expensive.
 A commercial hair dryer is handy for  general heating.
good for fitting bearings etc
be careful with one of these they can turn a bearing blue  in a very short time
When fitting the new nipples make sure to use a good anti-seize on the threads 
 Lub the seat with a little brake fluid 
Bfit