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Below is a pic of how the Caliper Piston Seal pulls the piston away from the Disc. When the Piston is moved inwards the Seal distorts. When you release your Brakes there is no hydraulic pressure and the Seal returns back to its shape and retracts you Caliper Piston a little.
So if your Caliper Piston Seal is stiff and loses elasticity it cannot pull your Caliper Piston Back.
Even if your Caliper Piston Seal is good it dose not have a lot of force to pull it back. Gunk under the between the Piston (I think this is most often the cause) and the Caliper wall can prevent the seal from pulling the Piston Back
If you for some reason let the Brake Fluid level get too low and some air got into the system and you later refilled the fluid. An Air bubble will also keep a Caliper Piston from retracting.
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84 300D, 82 Volvo 244Gl Diesel
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