There's no way the fluid in the rears would turn from heat before the fronts, as majority of braking (and thus heat generation) is done by the fronts. Furthermore, I doubt driving normally around town can produce the kinds of temps needed to do so.

The dark color likely stems from suspended particles from aging seals. If they're the originals like you stated, that puts them around the 7 year mark at the very least. Provided you don't have any scoring on the piston or excessive corrosion around the outer lip, caliper rebuild kits can be had for ~$25 bucks.

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