Trying to remember older movie

I believe its from the 80's.
It was about two brothers that run away from home or something to that affect.
One of the brothers, the younger one I believe, was convinced by his older brother that he was some sort of robot or cyborg.
At one point in the movie they are on a plane and the older brother is trying to prove that the younger brother is a robot and he tells him to chew on foil and if it sparks he is a robot.and it sparks.
I have been trying to remember this movie because it was one of my favorites growing up and I can't remember the title for the life of me!
It was about two brothers that run away from home or something to that affect.
One of the brothers, the younger one I believe, was convinced by his older brother that he was some sort of robot or cyborg.
At one point in the movie they are on a plane and the older brother is trying to prove that the younger brother is a robot and he tells him to chew on foil and if it sparks he is a robot.and it sparks.
I have been trying to remember this movie because it was one of my favorites growing up and I can't remember the title for the life of me!