Alex, the most frequent cause of the squeeling brakes on Volvos are after-market pads. Go and buy yourself a set of Volvo pads at the dealer (or from volvoparts.com, where they cost less) and you will get better performance and no noise. If you would like to give your break grease a try before you get the Volvo pads, go ahead. You put that stuff (it is silicone break pad grease) on the back of the pad, in other words between the caliper and the pad. This may or may not work. I've been reading this board for almost 3 years and 95% of all squeeling is due to after-market pads. Most people put them on to reduce black dust accumulating on the wheels, but get terrible squeeling. If your rotors are due for changing, DO NOT turn them, replace them with the Volvo ones as well. The brake systems in these cars are really sensitive to aftermarket parts and few people have good luck with aftermarket stuff. Volvo brake parts just work with no hassle.

Good luck.
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Vladimir. 1998 S70. Base, 5-speed manual.