Have you ever replaced the rubber brake lines? When old, they somtimes collapse and restrict the brake fluid. I suggest you first replace all the brake lines (buy Volvo genuine), flush and replace the brake fluid (DOT 4) and buy new brake pads (again, genuine Volvo). Then see how the brakes perform. Start with cheap parts and move on up to the expensive. Also, I disagree with the previous people who responded about calipers. If one side is bad, replace both calipers unless you know that the good side is much younger (e.g., you replaced it only a few months ago). Don't take chances with brakes, only go with the best and don't try to cut corners.