SteveB wrote in :

I just bought an air grease gun. I am amazed at how many zircs there
are on a tractor, and how many needed juice. Some wouldn't take any
grease. Do you just put a new one in, or is there a way to soak/clean
these?

Remove the zerk, and use pliers to hold it into the business end of your grease gun. Cover it
with a rag. One pull on the handle of the gun will blow everything out of the zerk.

Of course that won't help if the obstruction isn't in the zerk itself.