If your Brake warning light on the dash is not on, and you don't mess them up swapping them, then you can re-use them.
But, if the light is on, no matter what they look like, at least one of them has failed and will need to be replaced.
Lots of people don't use them, they're better aware of their car's condition than some drivers and just cable tie them out of the way. This cuts down on Brake expense a little and the car will never 'know' the difference.
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