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Hydraulic Braking System
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A hydraulic braking system transmits brake-pedal force to the wheel brakes through pressurized fluid, converting the fluid pressure into useful work of braking at the wheels. A simple, single-line hydraulic layout used to operate a drum and disc brake system is illustrated in Fig.

The brake pedal relays the driver’s foot effort to the master-cylinder piston, which compresses the brake fluid. This fluid pressure is equally transmitted throughout the fluid to the front disc-caliper pistons and to the rear wheel-cylinder pistons.

As per the regulations a separate mechanical parking brake must be incorporated with at least two wheels. This provision also allows the driver to stop the vehicle in the event of failure of the hydraulic brake system.

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In a hydraulic braking system the braking force is directly proportional to the ratio of the master-cylinder cross-sectional area to the disc or drum-brake wheel-cylinder cross-sectional areas.

Therefore these cylinder diameters are appropriately chosen to produce the desired braking effect. The wheel-cylinder cross-sectional areas of the front and rear disc-and drum-brakes respectively may be chosen to produce the best front-to-rear braking ratio.

Hydraulic fluid is incompressible provided there is no trapped air in the system. If air is present in the braking circuit, the foot-brake movement becomes spongy. In a hydraulic system the internal friction exists only between the cylinder pistons and seals.

The friction is caused by the fluid pressure squeezing the seal lips against the cylinder walls as the piston moves along its stroke.

A hydraulic braking system is suitable only for intermittent braking applications, and a separate mechanical linkage must be incorporated for parking brakes.

The hydraulic system offers the following advantages over the mechanical layout,

  1. This provides equal braking effort on all wheels.
  2. This requires relatively less braking effort to deliver the same output.
  3. This is a fully compensated system so that each brake receives its full share of the pedal effort.
  4. The efficiency of the hydraulic system is greater than that of the mechanical layout.
  5. This system is suitable for vehicles having independent suspension.
  6. It is easy to alter thrust on shoe because the force exerted on a piston depends on the piston area. The larger the area, the greater the thrust on the trailing shoe, so a larger piston can be used.
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