It's not so much the straight-line braking that's the problem, but the "tail wagging the dog" if the caravan is too heavy for the car. The caravan will look after itself in a straight line but, if you are ob a bend or a poor road, the caravan can push the car off line and, if you are unlucky/inexperienced, roll over. You need to make sure the caravan is within the 85% limit and loaded properly.