<P> For reliable, consistent brake system operation, brake fluid must maintain a constant viscosity under a wide range of temperatures, including extreme cold . This is especially important in systems with an anti-lock braking system (ABS), traction control, and stability control (ESP), as these systems often use micro-valves 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 . </P> <P> 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 . </P> <P> 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 . </P> <P> 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 . </P>

Whats the difference between dot 3 and dot 5 brake fluid
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