Get some Kroil, and give it a good soaking from the top side, and after a couple of days, tap the top of the stem gently with a ballpeen hammer to see if it will move. Alternately pry up, and tap, and if it moves at all, it should free up.

www.kanolabs.com

Kroil is one of the best penetrating oils I've ever used, even frees up rusted engines with a little time and work.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!