Clear Caches with cron with hook_cron

I try to clear the boost cache with a cronjob which triggers a small module like

function flushcache_cron() 
{  
  drupal_flush_all_caches();
}

Now I see the module is triggered by a cronjob but the Boost cache is not cleared. When I use the button 'Clear Cache' in the admin/performance section (admin/config/development/performance) the cache is cleared. Am I using the right hook or is it not possible to clear the boost cache this way?