What I have done on my restorations is remove them and soak them in naphtha or mineral spirits in my parts washer and move then around. It will loosen up the old grease. You can squirt some brake cleaner type solvent in there to help things as well. Lots of movement and some air pressure will help. I drill a hole in the middle and insert a grease zert as well. There are several kits on the market with seals. Once it it as clean as possible with solvent and air pressure, I grease it up and it’s good for another 40 years.