With some fluid removed and the top left off of the brake fluid reservoir, use a large pry tool to gently pry the pistons back in to the body of the caliper.

Always use the flat surface of the pry, never the tip, and try to push the piston back in as straight and evenly as possible.

Alternately, you can purchase the proper brake caliper piston compression tool from Mercedes...but this pry bar technique works if you are careful.

Repeat this on all four pistons. Watch the brake fluid level as your proceed to ensure it does not overflow the reservoir. Remove more if necessary.