Using dot 4 and changing every 2 years was arrived at by the Germans who studied the wet boiling point and considered it a safety issue.
http://www.mossmotors.com/SiteGraphics/Pages/Brake_Fluid/brake_fluid_long.html
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It is obvious that the reduction in boiling point is not going to be a problem if the brake fluid never gets that hot. That depends on how hot your calipers or wheel cylinders get, and that will depend on how hard the brakes are applied and how fast you were going. The brakes are going to get much hotter coming down a long descent on a mountain road than they are motoring through town. A brief period of aggressive driving and heavy use of the brakes can also elevate brake fluid temperatures tremendously. “Brake fluid can commonly see 150–200°C [300-392°F] at the brake calipers.” [14] The VACC suggests that 175ºC [347ºF] is the lower limit for “serviceable brake fluid”. If we accept that for the moment, it means that DOT 3 with ~2.5% water contamination and DOT 4 with 5% water contamination may boil in the calipers. The German TÜV (Technischer Überwachungs-Verein, or Technical Inspection Association), can be considered to be a Department of Transportation on steroids. They have established 180°C (356°F) as the minimum boiling point for fluid samples taken at the reservoir. At that point, the boiling point at the calipers and wheel cylinders will be close to 155 ºC (311ºF) due to higher levels of water contamination because of the proximity of the brake hoses. [54] The absorbed water reduces the temperature at which gas bubbles begin to form in the brake fluid. When these bubbles form, they turn the virtually incompressible hydraulic fluid into a mixture of gas and liquid, which can be compressed considerably, severely reducing the efficiency of the brakes. When this happens, the brakes feel “spongy” and the brake pedal travel will increase. It may be necessary to 'pump' the pedal to get the brakes to function. If the volume of the air bubbles in the system is equal to the swept volume in the brake master-cylinder, the brakes will stop working altogether. This condition is called “vapor-lock” and it translates into brake failure, because stepping on or pumping the brakes will have no effect whatsoever. [14]
If we overlay the boiling point minimums from the TÜV, it is clear that DOT 3 fluid will last about 6 months. At that point, the boiling point of the fluid in the calipers would be about 155 ºC (311ºF).
Not surprisingly, DOT 4 brake fluid is used in new Mercedes, BMW, Volkswagen and Audi automobiles because it will have a higher wet boiling point for an extended period of time.
