How do I create a link in the WP admin bar that purges all caches (W3 Total Cache)?

Question

You should use wp_nonce_url() to generate the URL with a valid nonce rather than hard-coding one, e.g. here’s the code from w3_total_cache itself that does this:

'href' => wp_nonce_url( network_admin_url(
        'admin.php?page=w3tc_dashboard&amp;w3tc_flush_all' ),
    'w3tc' )

You could also use a permissions check instead of checking if the user is ID 1, although I appreciate there are benefits to locking this down to a single user too.