Get ACF fields value before saving

First of all I’ve been posted , my question to the ACF support site, WP.stackexchange but all answer is something like “read the plugin doc.”. I know that I have to use THIS but I couldn’t understand the info from the plugin’s documentation and don’t know how to implement it with my function.

Here it is my problem:

I have a function that relay on acf checkbox field. It is working fine but I have to update the post twice, because get_field(‘checkbox_field’) returns the data after the other content. Example:

add_filter('wp_insert_post_data', 'my_func');
function my_func($post) {
    $pageID = 123; // certain page id
    $cp_field = get_field("custom_field", $pageID);
    if ($post['post_type'] != 'my_post_type'
        || $post['post_status'] == 'trash'
    )
        return $post;

    if(!get_field('checkbox_field')) { // The problem field
        // ... do something here
        return $post;
    } else {
        return $post;
    }
}

In short: I want when I do the check in get_field(‘checkbox_field’) to get it’s value before saving the post.

My last hope is in you guys. Thanks and sorry for my bad English.