Is this with the servo arm connected to the throttle/brake linkage? If so, remove the arm and see if the servo travels normally. It sounds like a binding in the linkage rather than a servo issue.

Throttle/brake linkages can be pretty difficult to set up properly and have a lot of possibility for rubbing/binding. Often there is an issue with the linkage screw through the servo arm hitting the top of the servo if not shimmed correctly.