the effect of water contamination on brake fluid performance was assessed, partly by measuring fluid temperature under a variety of conditions, but also by analysing fluid from two samples of cars to obtain information on water contents of fluids in cars on the road. water contents of one per cent in current brake fluids reduce the vapour lock point from around 250 deg c to around 180 deg c. further increases have progressively less effect. comparative tests to measure fluid temperatures were carried out on the track using nine different models of car. in addition fluid temperatures in the braking system of one car were measured under a variety of conditions on the road. under normal conditions maximum fluid temperatures were around 50-60 deg c. with a fully laden car or worn pads fluid temperatures were increased. a simulated binding rear brake raised the fluid temperatures to 143 deg c in the rear brake cylinder. during an extremely severe hill descent with the car fully laden 150-160 deg c was obtained. over half of a national sample of 1000 cars had three per cent or more water in their brake fluids, corresponding to a vapour lock point of about 145 deg c. water contents in fluid samples of different ages suggest that fluid changes at intervals of less than three years would produce only small improvements over present water contents. a firm relationship between fluid age and water content could not be established. fluid samples of the same age showed wide scatter in water content and some samples of relatively new fluid had high water contents. vapour lock is not to be expected in present day cars under normal operating conditions in the uk, unless the fluid is unusually heavily contaminated with moisture and either the car is at the end of a severe descent or some of its brakes are binding.(a)

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