Use a piece of emery cloth to "shoeshine" the tube for several inches. Even a little rust or dirt will make it very difficult if not Impossible to slide the ball back on the tube. Removing that grease fitting will allow the tube to slide back several inches if everything else is slicked up. Definitely unscrew the nut and try to avoid damaging the seal ring under it. A little oil on the tube helps, also.
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!