It is always possible to rebuild calipers, but they are also so cheap that it is silly to not just replace them. Keep in mind the down side to poorly operating brakes. It's not just your ability to slow down but also a sticky caliper will use up fuel and brake material thus costing your more money.

If you are compelled to rebuild, try soaking the piston to caliper area with a penetrating oil on the outside and on the inside through the fluid port.

We typically use compressed air to push the piston out. Hydraulic pressure will to the same as well. just pump the brakes with the caliper loose and perhaps just the outboard pad to catch the piston as it pops out.

Word to all, this type of issue is avoided by regularly flushing the system every one to two years. Hydraulic fluid absorbs water and thus corrodes over time as it becomes contaminated.

Best of luck.

__________________
85 300D 3 pedal. Current project.
83 300TD (need rear wiper assembly dead or alive)
84 300SD Daily driver
85 300TD almost 400k miles and driven daily.
98 E300D *sold
86 300SDL *sold and made flawless 10 hour journey to new home.