At full up or full down there is NO load, unless you have the linkage adjusted incorrectly.

You can put the retracts all the way down or up and remove the servo from the plane and they will stay down or up, unless vibration moves the over-center mechanism away from it's resting position.

If your servo has a load on it at either end, it will drain your battery quickly. Adjust the linkage so ti just makes it to each end without bottoming out.