I actually got to test my ABS in advertently today. was driving a bit fast, then started coming to a stop, so I started braking, a few little bumps cause the wheels to slip, the abs kicked in, and felt like it was grinding, but as the tires gained traction, it let up then it slip again and abs kicked in again.
Felt like it was grinding through the frame into my foot and I could hear it, but it wasn't there after I took off again and slowed down for my nexxt stop, which was smooth as always.
Listen to the people about the dirty sensors. many people have cleaned theirs and fixed their problem, so it can't hurt for you to try it. if that doesn't help, then look into getting them changed.
Not worth $700 if they only need cleaning.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.