Brake Fluid
Brake Fluid
A brake fluid meets the international standards set in the United States by the Society of Automotive Engineers (SAE) and Department of Transportation Federal Motor Vehicle Safety Standard (FMVSS).
The major characteristics of a brake fluid include :
(a) Low viscosity. The brake fluid must flow easily over a wide temperature range and be able to operate in very cold conditions.
(b) Compatibility with rubber components. Besides resisting corrosion of metal parts, it must be chemically non-reactive to the rubber seals etc. It must be non-injurious to the system.
(c) Lubricating properties. It must reduce friction of moving parts, especially rubber seals.
(d) Resistance to chemical ageing. It should have a long storage life and be stable when in use.
(e) Compatibility with fluids. It must be compatible with other fluids of its type.
if) High boiling point. Most braking systems use a glycerin-alcohol (glycol) fluid with additives to meet the required specifications. Because of the availability of a number of different fluids, some being vegetable and some mineral based, the manufacturer's recommendation should be referred before designing the system and also for subsequent refilling to avoid damage to rubber seals.
The major characteristics of a brake fluid include :
(a) Low viscosity. The brake fluid must flow easily over a wide temperature range and be able to operate in very cold conditions.
(b) Compatibility with rubber components. Besides resisting corrosion of metal parts, it must be chemically non-reactive to the rubber seals etc. It must be non-injurious to the system.
(c) Lubricating properties. It must reduce friction of moving parts, especially rubber seals.
(d) Resistance to chemical ageing. It should have a long storage life and be stable when in use.
(e) Compatibility with fluids. It must be compatible with other fluids of its type.
if) High boiling point. Most braking systems use a glycerin-alcohol (glycol) fluid with additives to meet the required specifications. Because of the availability of a number of different fluids, some being vegetable and some mineral based, the manufacturer's recommendation should be referred before designing the system and also for subsequent refilling to avoid damage to rubber seals.
Boiling Point of Brake Fluid
Glycol-based brake fluids are hygroscopic in nature and hence they absorb water from the atmosphere over a period of time. Presence of water lowers the boiling point and in extreme cases failure of the brakes takes place due to vapor locking. This situation arises when the temperature of the fluid in a part of the system rises above its boiling point so that the water in the fluid is vaporized. Once this happens, the elastic nature of the steam causes the pedal to reach its limit of travel before sufficient pressure is built up to apply the brakes effectively.
Because of the hygroscopic nature of most of the brake fluids, SAE and FMVSS specifications recommend the fluid to have a wet boiling point and dry boiling point in addition to the stated value. The 'wet' boiling point is the temperature at which a fluid containing 3 to 3.5 percent of water boils and produces steam bubbles. The 'wet' boiling point of typical brake fluids must be above 413 K. For safety reasons it is recommended to change the fluid in a brake system every year. A brake fluid absorbs about 5 percent water in this time period so that the boiling point is lowered to about half its original valve. Some new fluids have wet and dry boiling points of 453 K and 533 K so that the fluid renewal interval can be extended to 2 years.
Some special silicone-based fluids have been developed to overcome the hygroscopic problem, but these are costly and hence not commonly used. Brake fluids should be stored in sealed containers and should not come into contact with the paintwork of the car. If any fluid some how drips on to the paintwork, then it should be washed off with water immediately.
28.8.11.
Because of the hygroscopic nature of most of the brake fluids, SAE and FMVSS specifications recommend the fluid to have a wet boiling point and dry boiling point in addition to the stated value. The 'wet' boiling point is the temperature at which a fluid containing 3 to 3.5 percent of water boils and produces steam bubbles. The 'wet' boiling point of typical brake fluids must be above 413 K. For safety reasons it is recommended to change the fluid in a brake system every year. A brake fluid absorbs about 5 percent water in this time period so that the boiling point is lowered to about half its original valve. Some new fluids have wet and dry boiling points of 453 K and 533 K so that the fluid renewal interval can be extended to 2 years.
Some special silicone-based fluids have been developed to overcome the hygroscopic problem, but these are costly and hence not commonly used. Brake fluids should be stored in sealed containers and should not come into contact with the paintwork of the car. If any fluid some how drips on to the paintwork, then it should be washed off with water immediately.
28.8.11.
Bleeding the Brakes
Bleeding is necessary to remove air from the brake system whenever it enters. The main steps involved in the bleeding operation are briefly as follows, (a) Ensure that the reservoir is full with brake-fluid.
(6) Attach one end of rubber tube to bleeder valve and immerse the other end in brake-fluid placed in a jar.
(c) Open the bleeder valve and slowly operate brake pedal until air bubbles cease to appear. Close the bleeder valve as the pedal is being depressed.
(d) Repeat the above operation at all wheel cylinders, (c) Top up reservoir with brake fluid up to the mark.
28.8.12.
(6) Attach one end of rubber tube to bleeder valve and immerse the other end in brake-fluid placed in a jar.
(c) Open the bleeder valve and slowly operate brake pedal until air bubbles cease to appear. Close the bleeder valve as the pedal is being depressed.
(d) Repeat the above operation at all wheel cylinders, (c) Top up reservoir with brake fluid up to the mark.
28.8.12.
Faults
The main faults in the braking system and their causes are as follows :
Faults | Cause |
Pedal requires pumping | Shoes require adjustment |
Springy pedal | Air is present in the system |
Spongy pedal (pedal creeps downwards) | Leakage is present in the system, e.g. fluid is passing through main rubber cup. |
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