janl

I see by the forum threads that I am not the only Audi owner with this problem. My wife's 93 S4 with Brembo/Porsche calipers made some of the worst screeching/squealing noises ever heard outside a RR marshaling yard. When moderately applying the brakes, the noise was the loudest. During hard braking, some of the noise abated but driving in normal traffic would draw horrified looks of other motorists & pedestrians. Everything I tried made it better but 3 different pad compounds, a variety of anti-squeal backers & pastes, new rotors and a second set of slotted rotors, scuffing the rotors per Audi service bulletins only temporarily fixed the problem. A week later the noise returned. The largest improvement came from beveling the leading edge of the pads as all of my pads (PBI, hawk, Pagid were square cut with no factory bevel)which caused the pads to chatter when the brakes were applied softly. One weekend morning while watching the 2 guys garage TV show, the answer appeared.
If you look at a caliper on a bicycle for example, it is obvious there is a spring in the caliper mechanism to retract the pads from the rim. The only thing that retracts the pistons on automotive brake calipers is the torsion of the square sealing ring on piston. In its static state, the square seal sits “squarely “on the piston. When the brakes are applied, the inner bore of the seal grips the piston and as the piston moves, the seal twists or “coils up”. When the brakes are released, the seal uncoils and retracts the piston. The “2 Guys” explained that even though the seal is not leaking, the ability of the seal to act as a spring deteriorates with age decreases causing a number of problems including premature pad wear and noise. The guys at Paragon were very helpful and send me a rebuild kit. 95 % of my noise is now gone. One side note: Some brake noise is caused by the vibration of the pads chattering on the rotor. There are a number of companies making a mass damper to absorb & muffle these remaining noises. Some of the dampers are inserted into the piston bores while other designs have a weight attached to the end of the caliper attachment bolt. HTH, Jan L