[leegreen]

looking at the video, there might be a lot of friction on those i-bolts you used as guides. The cable changes direction quite a bit there. the cable will bite the bolt as it gets tight and the friction may increase dramatically. Test that theory with a dab of grease?

Trailing arm c10s had a similar guide to change angle of the cable, but the guide was free to pivot at the frame and so moves with the cable a bit. loosening your bolts might help isolate the issue as well.

If this does seem to be the problem look for 'parking brake pulley' online, they make little pulleys in a steel strap sheave that might work for you.

grease up that factory pulley you used for the same reason, if it is dry the friction will jump fast under load

Disclaimer: I have not put a c4 rear in anything but I did earn a living doing brakes for a while