drupal_set_breadcrumb() function doesn't work on "Add content" pages!
Ok, I just solved it! It's too easy! Using the below function in template.php file:

function mytheme_breadcrumb($variables) {
  $sep = ' &amp;gt; ';
  if (count($variables['breadcrumb']) &gt; 0) {
    return implode($sep, $variables['breadcrumb']) . $sep;
  }
  else {
    return t("Home");
  }
}


and using the arg() values and $variables['breadcrumb'] values in above function can enable you to check every pages path. Such as adding the below piece of code the above function:

  $breadcrumb = $variables['breadcrumb'];

  $is_add_content=false;
  foreach($breadcrumb as $key=&gt;$value){
    if(strpos($value,"node/add"))
      $is_add_content=true;
  }


References:

https://www.drupal.org/node/64067
https://api.drupal.org/api/drupal/includes%21theme.inc/function/theme_breadcrumb/7