Requirements :- The essential characteristics required of a braking fluid are discussed below:-
1. Boiling point :-
As considerable heat is generated during the application of brakes, it is the foremost requirement that the brake fluid should have a higher boiling point. With the continued research the brakes are becoming more and more efficient with more amount of heat dissipated. Therefore a high boiling point to avoid vapour formation is a must. In particular, the requirement is quite severe in case of high performance vehicles example racing cars and heavy transport vehicles. It is as such essential that the boiling point should not decrease with continued operation of the fluid. The boiling point should be generally around 250° to 300°C.
2. Viscosity :-
As the vehicle has to operate in varying weather conditions from very cold to very hot, the extreme conditions being when it is left overnight in hilly district, or kept in the sun for the whole Summer Day in the desert, it becomes imperative that the viscosity of the brake fluid should vary very little with temperature to keep optimum flow conditions. It is seen that contamination of brake fluid with moisture increases its viscosity marketly at low temperature.
3. Lubrication properties :-
The brake fluid should also be able to provide suitable lubrication to the components of the Braking System example master cylinder, wheel cylinder, otherwise they will wear out.
4. Effect on rubber :-
A number of rubber seals are used in the hydraulic Braking System. The brake fluid should not have any effect on these seal. A low quality fluid would either shrink or swell them. If the seals shrink there will be loss of pressure in the lines, leakage of fluid and entry of air into the system which would render the brake dangerous. Similarly if the rubber seals swell on contact with the brake fluid, there would be excessive pedal pressure requirement for break application, a higher rate of wear in the master cylinder and in case of extreme swelling, even the jamming of the brakes.
5. Corrosive action :-
The brake fluid should not corrode the metal components with which it comes into contact. If for example, it corrodes the main Piston in the master cylinder, the rubber seals will also wear out quickly, resulting in leakage of fluid any entry of air into the system.
6. Storage stability :-
Lastly, the brake fluid should have sufficient stability, say, for at least three years so that the same is not spoiled during storage.
Composition :-
In the beginning the brake Fluids consisted of mainly ethyl alcohol and castor oil. But the same had poor low temperature properties and low boiling point. The modern brake fluids contain polyglycols as lubricants and the glycol ethers as the diluents and possess all the properties listed above. Silicon based brake fluid has also been developed. This has wider operating range of temperature, does not absorb moisture and does not attract paint. However, it is more costly and causes more wear of the hydraulic system.
In case of emergencies, however, a mixture of 40 to 50% by weight of castor oil and 50 to 60% by weight of butyl alcohol could be used fairly satisfactorily. Alternatively, a mixture of 65% of rectified ethyl alcohol and 35% of rectified glycerine (by weight) may also be used. Any petroleum liquid should, however, never be used as brake fluid.
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