Why is wp_is_post_revision always false?

I am trying to save a revision of a custom post type. This is what I have to check if the post has a parent:

function save_revision( $post ) {

   if ( $post['submitted'] == true ) {
       $parent_id = wp_is_post_revision( $post['post_id'] );
       ...
   }
} add_action('init', 'save_revision');

The problem is that I always get false for $parent_id even though I know there is a parent post for that post_id.

Any help is appreciated, I have been trying to get this to work forever.