I just refurbished two clutches--one works but is noisy, the other works great and is what I left in the machine when I sold it.
There's no fluid inside, but the middle cylinder is filled with 325 ultra fine mesh iron oxide powder. The electromagnetic coil around the center (inside the metal housing) pulls the powder towards the outer perimeter and causes the two shafts to engage with each other using friction. Over time the fine powder can get past the seals in the bearings and cause them to become crunchy and awful. Any reduction of powder inside will reduce the FFB intensity.
If your clutch is noisy, you can pull the bearings out and replace them. If the FFB is weak, you can open the center "capsule" with the two shafts and refill it with 325 mesh iron oxide powder. Word of warning: working on these absolutely sucks, and you need to be careful not to get powder absolutely everywhere. If you get powder on the outside of the center "capsule" part, the clutch will lock up and you'll have to completely clean it all out and try again.
Hope this helps.
