A correct hook for saving meta boxes data

Question

Nothing happens when I do the var_dump() in a callback on post save.

Here are the hooks I've tried:

• save_post as shown in the codex

• pre_post_update as suggested here

• wp_insert_post_data found in this question

The code is rather simple:

function my_meta_boxes_save( $post_id ) {
    var_dump($_POST); exit;
    if ( ! wp_verify_nonce( $_POST['my_nonce'], plugin_basename(__FILE__) ) ) {
        return;
    }
    if ( ! current_user_can( 'edit_post', $post_id ) ) {
        return;
    }
    //some other checks here etc.
    //update_post_meta($post_id, 'meta_name', 'meta_value');
}
add_action( 'save_post', 'my_meta_boxes_save' ); // also tried the other hooks above

I need this to trigger every time the post is saved, including the cases when only the meta data is changed, and not the post content itself (this is where save_post fails).

Am I missing something? Any suggestions?

Update: @Dan it looks like save_post is triggered only if the post content was updated. I need to add about 10 meta boxes which might be edited without modifying the post content. In that case, when saved, save_post won't be triggered, my callback won't be called and the modifications entered in the custom meta boxes will be discarded.

Update: I am saving the meta boxes using the update_post_meta function. It was just removed from the code above so it looks less messy. I'm ok with users hitting the "Update" button, in fact, this is what I'm after. I do not need any ajax here or automatic save etc. I just need to make sure that my my_meta_boxes_save() callback is going to be called every time the users hit 'Update' or 'Publish' regardless of whether the post content itself was updated or not.

Right now I am debugging this, so I don't even need the meta save to happen - the var_dump() should just output the data, this way showing me that the callback was triggered, but it doesn't.