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Brake fluid is very hygroscopic. It absorbs moisture out of the air very readily. The components inside the brake system are not all made of corrosion resistant metals and the ABS system internals contains very fine precision fitted moving parts. If you don't change the brake fluid you risk some very expensive repairs later. The moisture accumulation inside system also lowers the boiling point of brake fluid and this can be disastrous if you or some other driver of that car, ever gets into a situation where the brakes are used to the point that the calipers get hot. Boiled brake fluid causes a loss of pedal that rapid pedal pumping won't fix. The gas inside caliper pushes the fluid back into reservoir and when you pump one pump stroke of master cylinder may not compress the vapors and apply brakes. When you release to pump brakes the gas pushes the fluid back into reservoir every time you release brake pedal---NOT GOOD!!!
Volvo has used DOT 4 (Volvo fluid is actually DOT 4+) for many years. Don't use DOT 3 as it might not be compatible with brake seal materials used in many European cars. DOT 3 also has a lower boiling point than DOT 4 spec fluid.
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