If one goes the grease zerk opener route,
IPA Grease Fitting Cleaner | Hoses, Valves Fittings| Northern Tool + Equipment
fill the tool with a solvent, diesel fuel, I have had about 50% success with this type of tool when I started to use a solvent type fluid in it,
and if it does not work then it back ot takeing out the zerk and using drill bits to remove the old grease, (no power tools, jsut twist with your hand to clean out the passage, and it it deeper and the passage is pluged at the opening of the passage internaly then take it apart to the place one can clean it up
and or if it pressure related being closed off, moving it so the grease can pass, (on one machine it is wore a little and the pressure seals off the passage, rotate the shaft a bit and it give clearance so grease will pass,
when I make a bushing I will usually make a ring cut in the pin so there is a place for grease to flow around the bushing and not dependent for just one location for grease exit,
and depending on how the machine is designed and it is working in dirt, dirt and grease can harden up like concrete, and some times mechanical removal of the old is about the only way to open it up, some times the solvent method works, and some times the locking grease nozzle will let one provide enough pressure to force it open,