I've been on a suspension related tear lately. Whats been working well on OLD crusty grease and grit encrusted parts and that includes zerks is to put them in a closed container along with a powerful grease cutting solvent overnight. Next day the curd just falls off.
On zerks a quick brush with a small wire brush, and a squirt of grease with a grease gun to clear out the now softened old grease and they're good to reinstall. Now if it's old hard grease in whatever you're greasing causing the problem then as Jerry said, heat is your friend.
On, and the powerful solvent I've found works best is gasoline. Always use it outside the shop in a no flame, no spark (including static electricity) environment.
RonR
