I have a new Vauxhall Meriva with a push button brake. Three times now we have pranged it when moving off on an incline. On one occasion I had to get out and wave the traffic behind to pass. Then, when there was nothing behind I revved hard and it still rolled back, which is very unnerving and I eventually held it on the clutch on the hill until the lights changed. Is this a feature or could it be a faulty brake?