and require very rapid activation. DOT 5.1 fluids are specified with low viscosity over a wide range of temperatures, although not all cars fitted with ABS or ESP specify DOT 5.1 brake fluid. For a faster reaction of the ABS and ESP systems, DOT 4 and DOT 5.1 brake fluids exist with low viscosity meeting the maximum 750 /s viscosity at -40 °C°F requirement of ISO 4925 class 6. These are often named DOT 4+ or Super DOT 4 and DOT 5.1 ESP. Boiling point Brake fluid is subjected to very high temperatures, especially in the wheel cylinders of drum brakes and disk brake calipers. It must have a high boiling point to avoid vaporizing in the lines. This vaporization creates a problem because vapor is highly compressible relative to liquid, and therefore negates the hydraulic transfer of braking force - so the brakes will fail to stop the vehicle. Quality standards refer to a brake fluid's "dry" and "wet" boiling points. The wet boiling point, which is usually much lower (although above most normal service temperatures), refers to the fluid's boiling point after absorbing a certain amount of moisture. This is several (single digit) percent, varying from formulation to formulation. Glycol-ether (DOT 3, 4, and 5.1) brake fluids are hygroscopic (water absorbing), which means they absorb moisture from the atmosphere under normal humidity levels. Non-hygroscopic fluids (e.g. silicone/DOT 5 and mineral oil based formulations), are hydrophobic, and can maintain an acceptable boiling point over the fluid's service life. Silicone based fluid is more compressible than glycol based fluid, leading to brakes with a spongy feeling. It can potentially suffer phase separation/water pooling and freezing/boiling in the system over time - the main reason single phase hygroscopic fluids are used. Corrosion Brake fluids must not corrode the metals used inside components such as calipers, wheel cylinders, master cylinders and ABS control valves. They must also protect against corrosion as moisture enters the system. Additives (corrosion inhibitors) are added to the base fluid to accomplish this. Silicone is less corrosive to paintwork unlike glycol-ether based DOT fluids. The advantage of the Citroën LHM mineral oil based brake fluid is the absence of corrosion. Seals may wear out at high mileages but otherwise these systems have exceptional longevity. It cannot be used as a substitute without changing seals due to incompatibility with the rubber. Compressibility Brake fluids must maintain a low level