1985 500sel Abs Light Is On And The Brakes(pedal) Pulsate Terribly/dangerously. 125000 Mile On Original Front Pads. I THINK!

HOW DO YOU CHECK AN ABS SENSOR??

CAN FLUSHING THE BRAKE SYSTEM HAVE ANY POSITIVE EFFECT ON THE ABS?

TO MY KNOWLEDGE IT HAS NEVER BEEN FLUSHED?

Any Help, Thanks
Thanks
John1