I don't know much about this specific problem, but most people either do not realize or do not care that they should be changing their brake fluid every few years. It is "hygroscopic" (I think that's the word) which means it sucks up moisture any chance it gets, and this moisture not only reduces its 'hydraulic' capacity, but also corrodes the insides of your braking system.
Regardless of what the ultimate solution to this problem is, I'd wager you've got the original factory brake fluid in there, and you should change it with new fluid, doing a complete flush (bleed all lines out at the calipers). This will at least let you know that you're dealing with the proper 'baseline' configuration. And who knows, it might help or fix your prob too, though I think that's less than likely. I've heard of stranger things though.
I don't know much about the S40 line, but I would expect it takes synthetic DOT4+ fluid.
In my 1998 V70 there is one rather large vacuum hose that supplies vacuum to the brake master cylinder, and I believe it runs from the master cylinder to the throttle body area, underneath all the intake plumbing. It would be prudent to check out that line.
Other than that I would suggest inspecting your lines for signs of 'washing' where the fluid might be leaking. You won't necessarily see any pooling under the car because the fluid evaporates fairly quickly, but it should leave a dark spot where it's leaking out.
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1998 V70 AWD Turbo 190k+
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