My only guess to this is the rubber flex line. Over time they can deteriorate and in doing so the i.d. of the line will become swollen and not allow fluid pressure to return out of the caliper.
When you did all of the above mentioned work, I assume you also went with new hardware (slide pins etc), correct? Galling (flat or irregular spots) or surface pitting on the pins will also cause the caliper to hang up like you're experiencing.
2001 Laser Red GT convertible; all of the usual bolt-ons.
1967 Notch; 200 I-6, C4 auto.