How bad do they have to be? I can probably see bad rotors from twenty feet out.... through the wheel. Depends on the problem. Knowing that MLs use up a set of discs atleast every second set of pads helps in the remote analysis. Stick your hand through the wheel and feel the lip at the edge of the disc (that an experienced tech can see). The disc is flat when new. There is a limit to how thin the rotor can get and when you know where they start and can see more lip than the wear tolerance, it gets real easy.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician