Automobile braking systems work on hydraulics and a lubricating fluid called brake oil is necessary for them to function. Brake oil has evolved to ensure better operation and reliability of the system.
DOT 3, DOT 4, DOT 5 and DOT 5.1 are the types of brake oil listed corresponding to their boiling points as they work in very high temperature environments. DOT 3 and DOT 4 brake oils are most commonly used.
Boiling points for common brake fluids
The boiling point of brake oil needs to be high to avoid vaporisation, as vapour is compressible and will hamper the effectiveness of a hydraulic system. Over time, due to moisture content in the atmosphere brake oil absorbs moisture. The boiling point of brake oil after absorbing moisture is known as its wet boiling point.
DOT 3, DOT 4 and DOT 5.1 are able to absorb water to an extent (hygroscopic) under normal humidity levels. DOT 5 is silicon based and cannot absorb water (non-hygroscopic). DOT 5 brake oil is not compatible with anti-lock braking systems. Silicone brake oil is used in very cold climates.
Brake oil must be changed every two years as the oil gets saturated with moisture. The moisture content can also corrode seals and metal inside the braking system. Old brake oil is flushed out completely and replaced with new oil.
Ensure an adequate level of brake oil is refilled according to the marking on the container located in the engine bay. If the level drops, top up with the grade of brake oil recommended by the car manufacturer. Do not use brake oil from an opened bottle as it will have absorbed moisture from the atmosphere.
Wash off any brake oil residue that might have spilled in the engine bay while refilling as it can corrode the paint.
Castrol, Petronas and Mobil are the best brake oil brands. A litre of branded brake oil costs about Rs 300.