Brake sensors; W123

My '79 300TD recently started seeing the "low brake pads" light. It started happening right after I had it in the shop because of some suspension problems I was having (broken sway bar mount). The light
"pulses" while I'm braking, and sometimes is on while the brake pedal is not depressed and sometimes it is off. The pulsing is related to the speed of the vehicle.
I know for a fact that the back pads are huge, they looked almost-new when I had the back wheels off recently. I haven't actually inspected the front pads yet but I suspect it's related to the work done at the shop (they were sloppy in a number of areas, and the fact that it appeared literally the day I got the car back makes me suspicious).
So, I'm curious. How do these sensors work? Are they adjustable, or do they just bolt in and that's that? Does this lamp on the dash mean anything besides "need new pads?" (I.e. I don't know what light would illuminate if my brake fluid were low, which it isn't.)
If I do need new pads, do people recommend any particular brands? The sensor light pulses, which makes me think the rotors may not be true, but I don't notice anything when I'm driving (as opposed to any American car I've ever had in which the rotors would noticably warp within about a thousand miles of being replaced). Oh, when I do the pads, should I replace the sensors too? I see Autohaus sells the sensors individually for like $1.50 which implies they're pretty highly disposable.
thanks a lot, I appreciate the help everyone's given me here. -tom!
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Is it the symbol that looks roughly like this ((o)) ?
If so the mechanism is simple wires embedded in insulation. The wire inserts laterally into the pad and when the shoe is worn almost to the rivets the wire is grounded and voila! the light comes on. Pretty simple.
My guess is that you are on the bleeding edge of the life of a pad and that is why the light is intermittent. You might also have a slightly warped disk which might also cause an intermittent ground.
OTOH they might have just pulled a wire out and left it dangling too :) I think the 123 has two wires on each side, one for each pad.
I expect you need new rotors and shoes in the front though.
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me wrote:

Yeah, thanks.

Oh, huh. Ok, that's dead-simple then, and pretty much unambiguous and almost free of user error. :)
So this indicator doesn't indicate low brake fluid, that function would be handled by the lamp that comes on with the parking brake like many other cars?
thanks, -tom!
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This (on my 126s) was an yellow/amber light. The brake failure light was red and said BRAKE. Not sure about the 124s, but I am guessing its the same. You need to get a manual.
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me wrote:

Yeah, the late-model 123s changed this to an amber light too, but it was red in 1979 (assuming the coloring isn't on the bulb itself).
Thanks for your help.
-tom!
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