Uh, the only way I can think of a drum cylinder being bad is if it's leaking. So I guess you could just take off the wheel and see if it's wet with brake fluid around the cylinder.
How does the vehicle stop. Does it feel like it's braking correctly? I never really put too much faith in some of those factory "warning" lights. Do the brakes feel firm when you're driving down the road and then brake? If you brake hard, do any of the rear wheels lock up?
Is it the brake warning light or is it the light that tells you one of your brake lights or tail lights are out? Your car is an '83? If you're looking at the console where the diagram of the car is and shows all the lights then that's just means one of your lights have gone out. I dunno, there's a lot of things you can check. Sorry if I wasnt' any help
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