Jobs like this take patience... penetrating fluid and time can work wonders.

Barring that... WD40 and a torch in a well ventilated area can also do wonders.

Spray the WD40 into the seized part and then heat it with the torch being careful not to burn the frame, cable housings, or yourself... the heat causes the WD40 to burn off and in doing so can often break the hold those parts have on each other.

Have used this on small jobs and for extracting 3 inch bolts from oilfield equipment that might have been in the field for months or years.