I use a 5' bar on the centre locks on the Plus8 and sometimes the front wheel will rotate and I either chock the wheel or get someone to push the brake pedal. I don't overly tighten them either they do seem to tighten up as per the theory.
2009 4/4 Sport Henrietta 1999 Indigo Blue +8 2009 4/4 Sport Green prev 1993 Con Green +8 prev