I don't drive my C4 quite as much as I used to and I began to notice a
strange pulsation on braking.  I thought something such as warped rotors
were the problem.   I mentioned it to my local Chevy dealer who told me the
rotors had rust spots on them from the metallic pads.  He said that it was
very rare for a wheel to stop in exactly the same position each time, so
rust spots developed in other spots creating a series of rust spots that
were felt as pulsations on braking.

The car stopped fine, and in a straight line, but the annoying pulsation was
there.  A clean up of the rotors with sand paper sorted it all out.

The dealer, second generation, same location 30 years, tells me that cars
that sit on the lot for any time at all have the  same problem as the pads
come to rest on the rotors and that in doing so they form rust spots that
are felt as vibrations.  It is necessary to give some pretty hard braking to
clean those spots up in order for a customer to not believe that something
is wrong the brakes. That, or they have to take the car into the shop, pull
the wheels, sand the rotors, put the wheels back on the cars to satisfy the
customers that the brakes are okay.

Joel Jacobs