It is very difficult to seal a drive shaft encased in any fluid with a rubber gasket (I assume that is how any fluid trainer is built other than a Kurt Kinetic). Saris is a good company though and should work with you. I would guess that when they leak, you just have to replace the rubber gasket (probably by sending it in and having it rebuilt.

The design of the Kurt Kinetic is brilliant. The guy that thought of that magnetic coupling system is pretty smart.