Open the brake fluid reservoir and then push the caliper back with a large flat screwdriver. It takes some time and effort to push it back. Be careful not to damage the rubber.

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1979 Black on Black, 300CD (sold), 1990 Black 300SE, Silver 1989 Volvo 780, 1988 300CE (vanished by the hands of a girlfriend), 1992 300CE (Rescue).