Comma experts tested nearly 700 motors and found half had some form of defect - a third being potentially lethal. The cause was contaminated brake fluid. As the liquid ages, it absorbs water and can 'boil' at low temperatures, causing sudden brake failure. Spokesman Mike Bewsey said: "There is no warning before it happens." Comma wants brake fluids checked annually. Our test of cars' stopping distances in Issue 917 highlighted the dangers of faulty brakes.