Stuck rear calipers

[Jagernauth, Stephen, RTLSL]

Searching the archives, I found little on this subject.  Did a Google search on the mother list as well.
I'm replacing rear rotors/pads since they are down to minimum.  I'm changing fluid and fixing stuck e-brake cable/rusted shaft
as well.   (Just got done with suspension bushings).
Pistons will not retract at all, I don't have a special tool to fit into the piston grooves.  15" Channel Lock is doing nothing.
Caliper rebuild time?   I can take the calipers off and stick in the vise if that will help.  Do I need to crack bleeder valve to
relieve pressure, never had to do that so I suspect calipers are really frozen.   Need to pass inspection so I can get 20V
back on the road and tuck the 28 year old daily-driver 911 into garage for winter.
If I have to rebuild calipers, do I need special tool to get them out?  Home-brew solution?

Advice welcome.

Steve Jagernauth - Easton, PA
'91 200q20v
'74 911/3.0 Targa - street
'74 911/3.2L coupe - track
'93 GMC Sub - wife's/tow