This article applies to any Z-car clutch that can be disassembled.
The clutch works by shearing a viscous fluid, the drag from which couples force from the driving shaft to the driven hub. The clutch consists of two sections, a fixed drag section that provides a minimum level of torque and a variable drag section controlled by a bimetal thermostatic coil visible from the outside.
The most common failure, indeed the only failure I've ever seen, is from the viscous fluid leaking out. Typically this happens over several years and is not noticed because it doesn't leave a nice wet spot. If the unit goes completely dry, the clutch will sometimes seize. More common is that just enough fluid leaks out that the variable side no longer functions. The fan still turns at idle but there is insufficient drive to cool the engine.
Generally the seals are still OK and don't need to be replaced. Rebuilding consists of opening the housing, cleaning it and refilling with fluid. Opening the housing involves removing the clutch from the fan and removing the 4 6 mm bolts that hold the housing together. Then gently pry the housing open. There is an O-ring so a little force is needed. There is nothing to fall out and no springs to surprise you :-)
Pour out any remaining oil and clean with brake cleaner or something similar. Then completely fill the shaft-side housing with 90 wt gear oil and add about 5 cc of laquer thinner. This slightly swells the rubber in the seals and makes them more leak tite. Make sure all air bubbles are eliminated. Then assemble the housing, replace the bolts and the job is finished.