Before you do that, spray the fitting with a penetrating oil spray. You
can use WD-40 (which does work somewhat, but it is more of an all
purpose lubricant and water displacer, hence the original meaning of the
name WD). A pure penetrating oil like "Nuts Off", CD2, or any of a dozen
other brands sold in hardware stores will probably be more effective.

Thanks. I will try WD-40 first. If it is not good enough, I will get
the pure penetrating oil as you have suggested.

Jay Chan