I bought a grease gun to lube up my 74 FJ40 (all stock). I notice that I can put the attach the gun to the zerk (not sure of the spelling) but once on it will not come off. I have to yank the daylights out of the thing to remove it. Are there different size fittings? If so, what is stock on this unit? And, how can i tell what size my gun has? Thanks.