The rear brakes on my 1990 300CE make a very loud squealing noise when applied any where below about 20 mph. I have previously changed rotors, calipers and pads several times which only cures for a relatively short time and then the squeal comes back. I have used 3 different pad manufacturers, all after market. None of the pads sets came with shims and the pads that were on the car before I did any work did have shims and did not make noise. I'm thinking maybe I need to get shims (or a pad set that has shims), although I am also going to try cleaning the ABS sensors (per EC93SE from 2-24-2006). Any thoughts/advice would be appreciated.