Sounds like a siezed piston. This occurs typically from two causes. Either the piston is not aligned perfectly with the cylinder bore or there is corrosion on the cylinder walls behind the piston preventing it from going into the bore.
Brake fluid is hydoscopic, meaning it attracts & absorbs water. If the fluid is not changed at the recommended intervals, any moisture in the fluid will cause corrosion in the brake lines, cylinders, etc. The solution is to remove and rebuild or replace the caliper. A flush of the brake fluid would also be prudent.
The rear brakes on your '02 s40 are slightly different from the front. On the front, you can press the piston in. On the rear, the piston needs to be screwed in. Notice the slot going across the head of the piston? There is a special tool that looks like a small block and sits on top of piston and a small piece sets in the slot as the block sits on the piston. Turn clockwise to press in. You can use pliers of some sort but need to be careful because you can rip the rubber. You need to turn the piston in clockwise direction to push in the piston
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