This is a really common problem on c3's & c2's with rear disk brakes. When the cars were orignally manufactured the rear hub & rotor were machined together, after the years go by and the rotors are pulled off & then not put back in there original location, or maybe replaced you end up with rotor run out. The reason this effects these cars is because the original style seals were designed to keep brake fluid in, they were never designed to keep air from getting in because when the rotor runs true the problem doesn't happen. The brake pads on a c3 caliper ride on the surface of the rotor all the time & if you have rotor run out it will cause the pads to move in & out which in turn sucks air into the rotor, air that probably contains moisture which in turn rusts out the sleaves in the caliper. The more run out, the quicker the problem happens. You'll need a dial indicator and a shim pack that you can purchase at Nappa, Rock auto, etc. there are numerous articals on the net that will walk you through the process.