Uni Joints - Grease Fittings

G'day Stu,
Not sure what you are driving, but I'll assume it is an early model vehicle (80's - 90's). There could be a number of reasons why the grease nipple appears blocked. They range from what grease has been used during servicing in the past, and temperatures and terrain that the vehicle has endured, during this time, and also the frequency that the services have been carried out. Quite often some greases will harden to the extent that they set like hard window putty. No amount of pressure will move this 'putty' very far. If this happens in the grease nipple, this 'Hard Gease" will only be moved a slight fraction back to take up the small void behind it, and the only fresh grease that will get in will be in the top entry part of the nipple near the ball bearing. (hence the grease being forced back out of the front of the grease gun - I have encountered this problem a number of times in the past, caused mostly by dirt being forced back into the nipple. The person who carried out the servicing, didn't clean the nipples first before hooking up the grease gun.) My advice is to unscrew and replace the nipple for a cost of around $2.00 if you are concerned. Places like REPCO, AUTOBARN, SUPERCHEAP, etc should be able to supply you with the new nipple. Just beware, as these come in different head and also thread sizes. Hope this helps somewhat, cheers Lyndon.

I run into this problem quite often on my old backhoe. It has so many grease nipples I've never added them up and got the same figure, but it's well over 50!
I'm too mean to replace nipples until they're physically destroyed so I just unscrew them, put them in the grease gun and force grease through. Generally this clears them as the obstruction can pop out.
Not sure if it's fair to blame previous servicing for the blocked nipples. I've had this backhoe for years and I always clean the nipples before greasing. Still get the odd one that blocks up.