On my 86 300sdl, it turned out that one of the wires in the wiring harness of the brake sensor had worn out and cracked. It was where they clip into the plastic housing that holds them in placeon the front suspension. When I removed the ABS sensor and wiring, the wiring literally crumbled. I located a nice used driver's side sensor and wire and all has been well since.
Check those wires!

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John
1986 300sdl
1985 380se