Clutch thrust bearing

Hi,
went riding a bit this weekend and just as I pulled up back home I felt some wobbly feedback through the clutch lever. took out the clutch operating mechanism and found the thrust bearing was buggered. I packed it with red high temp grease the last time i packed it. when i took it out it was all rusted looking and smelling a bit of gearbox oil. also some outside damage on the thrust piston, probably overheating when the bearing failed.
My Q, why would this bearing fail like this and where does the rust come from? I dont want to fit a new bearing if it will fail for the same reason. any thoughts?
I will pull the gearbox this weekend to inspect the thrust pin.

Which thrust bearing are you talking about ??

There are two types. Post 1986, pre 1986.

But quite a few people i know have fitted the earlier type on their later bikes.

1981 G/s

I remember packing the bearing with grease and found grease in the rubber boot when I removed the clutch mechanism. the bearing looked pretty dry though. There's some hefty marks on the piston and I think I should probably replace it.
How much grease do you guys normally pack in these bearings? and what type?

Are you guys aware of an upgrade that came with the piston/bearing unit?
BMW had a different part than the one I had, but it seems it might be a single unit replacement for the one I have. BMW couldnt tell me if it is a replacement though.

The later part fits and was classed as an upgrade, however the new part is a one piece item and costs about three times the price of a replacement bearing. The new part is also prone to binding in the gearbox housing resulting in a slightly rough clutch action - unless you spend a little time taking it down in size. Not everybody has this issue though.