// ADD => WORKS
if ( $content->create( $content_type, $data )->isSuccessful() )
// works -> adds to free table and adds to cck_core table
// DELETE => WORKS 50%
$content->delete( $pk ); // deletes from free table and DOES NOT deletes from cck_core
Did you try deleting items of an existing content type? I also have content types with free storage created some years ago and they work fine when deleting. The problem only happens when I create a new content type. One text field with free storage, one submit button and a list type with a "delete button".