Rear Caliper

["Weeks, Chet" <Chet.Weeks@standardregister.com>]

Listers:
Could use some input on the rear caliper care and feeding. The car is an 89
90S-FWD. I think the calipers are Girlings although I am learning the the
modl 89 year was a stressful one for Rube Goldberg -chief Audi designer. I
think they changed the braked design within the model year (at least that's
what the svc manual sez). The failure mode was the brakes were frozen- ie
the caliper did not retract to allow the rotor to rotate freely. This
created substantial heat and cause for me to take things apart. Here are a
few points of potential interest:
-I got the caliper off and tried to push the brake piston into the housing
-NG -it is immovable. 
-I turned the piston clockwise and it rotated ok and seemd to retract the
piston a bit - at least enough to get the caliper back on the rotor. 
-I noted that the parking brake when actuated works fine, and the brakes
work fine and retract by the brake pedal. However, when the parking brake is
released, the lever on the caliper does not fully release and leaves brake
pressure applied. I lubed the area substantially -Rube's favorite repair
technique (next to duct tape) but the lever still doesnt retract competely.
Here are my quandries:
1)Should the piston with force applied return inside the caliper housing
??If yes, what could be holding the piston. 
2)Is it worth taking the caliper apart and rebuilding relubing -esp the
parking brake area ???
3)Does anyone have any drawings or description of the internal design of the
caliper -the svc manual has nothing. 
4)Is a rebuild kit avail anywhere??
5)What are the tricks to a rebuild. 

Many thanks,