Parking brake 4000 quattro

[Ben Swann]

I am in process of checking/installing calipers to fix 4000 quattro rear caliper that is not pushing the piston out when the lever is turned - either by hand or with the cable.

I took a good caliper off a 5000 q - worked fine before taking it off.  After installation and bleeding, the piston doesn't push out anymore.  Now I have scrounged up several of these, and they seem to have the same problem.  Perhaps something with removing the piston and cleaning and installing good seal has  made one stop working.  

What is causing this?  how come the parking brake portion of the caliper stops working, even though the lever moves freely?  Moving the lever results in no or barly perceptible movement of the piston.

Can this be fixed?

Ben