I suggest that the servo is getting stuck in the engaged position because:
1) Electrically, turning off the switch should have disengaged it. Failure to disengage at that point would tend to indicte that it is NOT an electrical problem, and point towards a mechanical problem.
2) Hitting the brake SHOULD dump the vacuum to the servo. The servo is operated by a vacuum diaphragm much similar to your power brake booster. Dumping the vacuum should cause it to immediately release the throttle. There is a vacuum dump switch on the brake linkage that you should be able to check out.
These two items pretty much rule out a vacuum or electrical problem. My vote is to replace the servo. Also check for any binding or restriction in the linkages. Overall, I would say that with 99% certainty, your problem is mechanical, and the prime suspect is the servo.
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