Professionals know there is no way to free up a cable once it has become sluggish or rusted. The only reliable repair is to replace them. Sooner or later every inexperienced mechanic has a car come back on a tow truck with a cable stuck in the applied position that he tried to free up previously.
The only thing you can do is to try to fully retract the cable, then NEVER use it. You might get one emergency stop out of the other side, but then you'll be stuck there.