Had this problem on one side with the family van. One of the calipers had similar difficulties with the piston but eventually the new pads went in ok. Upon initial drive I could smell the burning and I felt one wheel was overly hot to touch. I wound up replacing the bad caliper and brake line and problem was solved. I had heard that the brake line could be a factor with the release mechanism and it was cheap so I did that too...
1993 300e-2.8 - gone now <sigh>
"Do not adjust your mind, it's reality that's malfunctioning"