Some MB cars use "floating" calipers and some don't. Floating calipers have one half fixed and the other can move. If the caliper is corroded externally it may be difficult for the caliper to float. You may free it up by a good cleaning, wire brushing, etc.

Otherwise, if the piston is stuck, it's normally because the seals have gone and the cylinder bore is corroded. Time for a new/rebuilt caliper.

Rgds,
Chris W.