The zirk nuts on the proaction are welded in and they do not have balls in the zirk believe it or not. correct on the transfer rods as they are threaded, the rest are all open unthreaded zirks. I take my skids out for service most years and still find plugged ones. SInce their is no ball try a needle to open it up. I also found that having a friend hold the grease fitting to the zirk then slowly pushing the grease in will work also. I noticed since i changed to amsoil synthetic grease it doesnt do this as much, and its blue also (bonus) LOL! Sometimes the only way to unplug is to take it apart and clean old grease out. Depending on where (always the lower rear pivot arm for me) you can take it apart without removing the skid. I did see at the local hardware a devise to place on your grease gun that will unplug the zirk. It was only a few bucks maybe worth a try.