1986 k10 brake light help
 

I have a 1986 k10 with brake light trouble. All my other lights work, when the truck is not running the brake lights work in every gear. When i start the truck up all the lights still work but as soon as i start driving just the brake lights go out. I put the truck in park still running and no brake lights. I shut the truck off and then push the brake and they work. Any ideas?
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