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BTW, there are 2 reasons for the recommendation of this fluid. One is the fact that it is resistant to absorbing moisture from the air, while Dot3 is not. Second, it doesn't attack the rubber used in the braking systems of many european cars like Dot3 will.
Both of these recommendations are technically untrue, though it's a rather moot point in our context. Both DOT 3 and DOT 4 will absorb moisture. There's just no way around that when you've got flexible hoses at four points and many different pistons whose travel is open to the moisture in the air. In fact, it is H2O in the lines that is more of a concern than actual air in the lines when you do your periodic bleeding. If your system hasn't been opened for any reason, after a couple of years you won't have any air in the lines, but you will always have water that you need to get out, since water compresses and starts to rust and corrode your hydraulic components. This is true just as much for DOT 4 as DOT 3. And it's not DOT 3 that isn't good for the rubber of some cars. It is no different from DOT 4 in that regard; it is DOT 5 that you are probably referring to.
Chemically, the DOT 3 and DOT 4 formulations are practically identical. Virtually the only difference between the DOT 3 and DOT 4 specs is that DOT 4 has a higher boiling point than DOT 3. It is extremely rare that the hydraulics in a Volvo would ever exceed the boiling point even of DOT 3, so Volvo's recommendation of DOT 4 can be seen as another example of Volvo's ultra-conservative stance on all things that effect safety. I'm not saying don't get DOT 4; definitely go by Volvo's recommendation. But if you need to top off with DOT 3 in a pinch, don't lose sleep over it.
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