I had a similar problem after my pads were replaced with the squealing and the ABS activating at very low speeds. It was cured by cleaning the metallic 'fuzz' from the ABS sensors and taking a little more material off the rotors. The squealing was VERY annoying, almost earsplitting at times.

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2002 500SL(Black/Black)
1992 300SE(Black/Black)
1992 190E 2.3(White/Navy)
1999 VW Jetta TDI(Pearl Green)
1986 SAAB 900 Base 3 door(White)
1983 Porsche 911SC(Chiffon)
1977 VW Scirocco(DiamondSilver)