My partner has an 8120 and the roller ball is ok, rolls fine but the button is a nightmare. You have to press the button really hard to get it to work and sometimes it doesnt work at all.
I have a spare 8100 so i took the roller ball from this and put it in the 8120 - still the same.
Replace the trackball dome. Its the little gold square under the TB. They only cost a couple of bucks and can be easily replaced with a pair of tweezers. You can find them in the CB store.
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