The rubber eventually dies of oxidation, and the pistons won't retract, or worse, stick out and have to be forced back. They should slide back into the calipers (cap off reservior!) with light pressue from a small prybar or large screwdriver. If they won't push back smoothly and fairly easily, it is likely the dust boot is cracked through and there is dirt in the piston bore. This dirt will cause the piston to stick (no clearance), and a hard piston seal won't help much. Rebuilds are easy, and you will then have brand new brakes!
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!