action hook save_post not working for custom post type

Hi everybody,

I am trying to use the action hook ‘save_post’ for a metabox within a custom post type. Using the ‘publish_post’ like this:

add_action('publish_my_custom_post_type', 'save_product_data');

works fine, but this

add_action('save_my_custom_post_type', 'save_product_data');

doesn’t. Of course I can work around this like this:

add_action('save_post', 'save_product_data');
function save_product_data(){
	global $post;
	if ($post->post_type == 'my_custom_post_type') {
	update_post_meta($post->ID, '_price', $_POST['_price']);
	update_post_meta($post->ID, '_product_number', $_POST['_product_number']);
	}
}

But I would really like to understand why the first hook is working but the second one isn’t. Any ideas?

Thanks a lot