Lesson 1/Learning Event 2
cylinder bore against each piston to prevent the escape of brake
liquid. There is a light spring between the cups to keep them in
position against the pistons. The open ends of the cylinder are
fitted with rubber boots to keep out foreign matter. Brake fluid
enters the cylinder from the brake line connection between the
pistons. At the top of the cylinder, between the pistons, is a
bleeder hole and screw through which air is released when the system
is being filled with brake fluid.
On some vehicles, a stepped wheel cylinder is used to compensate for
the faster rate of wear on the front shoe than on the rear shoe.
This happens because of the selfenergizing action. By using a
larger piston for the rear shoe, the shoe receives more pressure to
offset the selfenergizing action of the front shoe.
If it is desired that both shoes be independently selfenergizing, it
is necessary to have two wheel cylinders, one for each shoe. Each
cylinder has a single piston and is mounted on the opposite side of
the brake backing plate from the other cylinder.
So far, we have discussed the parts needed to make up a hydraulic
brake system. Now let's see what happens to these parts when the
brakes are applied and released. Let's assume the master cylinder
is installed on a vehicle and the hydraulic system is filled with
fluid. As the driver pushes down on the brake pedal, linkage moves
the piston in the master cylinder. As the piston moves inward, the
primary cup seals off the bypass port (sometimes known as the
compensating port).
With the bypass port closed, the piston traps the fluid ahead of it
and creates pressure in the cylinder. This pressure forces the check
valve to open and fluid passes into the brake line. As the piston
continues to move, it forces fluid through the lines into the wheel
cylinders. The hydraulic pressure causes the wheel cylinder pistons
to move outward and force the brake shoes against the brake drum. As
long as pressure is kept on the brake pedal, the shoes will remain
pressed against the drum.
When the brake pedal is released, the pressure of the link or pushrod
is removed from the master cylinder piston. The return spring pushes
the piston back to the released position, reducing the pressure in
front of the piston. The check valve slows down the sudden return of
fluid from the wheel cylinders. As the piston moves toward the
released position in the cylinder, fluid from the master cylinder
supply tank flows through the intake port and then through the
bleeder holes in the head of the piston. This fluid will bend the
lips of the primary cup away from the cylinder wall, and the fluid
will flow into the cylinder ahead of the piston.
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