Calipers or Hoses
I agree with the previous post, the caliper may not be releasing properly, which maybe due to the piston, but also check the emergency brake cable to ensure it is releasing properly. Also, the brake hose, attached to the caliper maybe rotting on the inside, and restricting the fluid flow. Have you ever flushed your entire brake system? The MB manual recommends replacing brake fluid annually.
1989 420 SEL