Sticking brake caliper on 380SE

Tried to replace front brake pads on my sisters 1984 380se today.
Passenger side completed, but had difficulty compressing the
caliper pistons back in, to make room for the new pads.

However, I could not budge the drivers side pistons. I DO NOT
have the special factory tool for this, just careful prying with
various implements.

What is the trick for this ? Or do I have to replace the caliper ?
If so, any special tools required for this, other than a torque
wrench ?