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I don't know if this is the case for Volvos, but I have heard that on some cars, once the brake failure light comes on, you have to manually re-center, or replace, the pressure differential switch to get the light to turn off.
This was the case for older/vintage Volvos, until they started using a non-locking electrical piston switch in the pressure differential switch (aka Junction Block). I believe the change was made to the non-locking switch in 1975 with the start of the 240 production. All of our 200/700/900 and newer vehicles should re-center and reset themselves during a firm applicaiton of the brakes once all air has been removed from the lines. The prior vehicles had a detent in the sliding piston that would catch on the spring loaded electrical switch contact. Resetting the brake failure light required unscrewing the electrical switch where it mounted into the pressure differential switch, recentering the piston (press on the brake pedal and hope you've got all the air out), and then screwing it back in. For a picture of the older Junction Block and the switch system used in it, check out a previous post: here.
I'm glad my brake line metal composition error was pointed out. It's also further testimony of Volvo's commitment to the longevity of their vehicles. However it's still a good idea to flush the system every two years since there are steel components in contact with the brake fluid (the largest ones probably being the caliper channels, and in close proximity to the pistons). Just think of a brake fluid flush as being the final maintance step after you've changed your engine coolant (since both are supposed to be done at 2 year intervals).
God bless,
Fitz Fitzgerald.
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'87 Blue 240 Wagon, 251k miles.
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