Getting the piston out and then trying to get it back in is not something I would recommend unless you know what you are doing.
I am surprised that the piston is frozen in the caliper in a 1997 Mercedes. Something doesn't sound right.
I got this picture from the DIYer section, does yours look like this caliper?
BTW, prying against the rotor with a screw driver to push the pistons back into the brake caliper is not a good idea in my book.
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)