I have removed a number of old press-in grease jerks with ViceGrips. Usually a slight twisting motion will free them. There is a small flat part on the zerks that a ViceGrio can clamp onto. I have never had one crush to where it wouldn't come loose. Another method I have not tried, but would seem to work if ViceGrips don't work is to grind off the top o the zerk and use a tap and thread it into the zerk hole and clamp ViceGrips to the tap and drive the Jerk out. I have used this technique to remove distributor bushing. Most original jerks, I have found, are plugged, discolored and/or rusted and need to be replaced.
Rusty Nelson