The self-adjusting brake mechanism operates
only When the brakes are applied while the car is moving rearward
and only when the secondary shoe moves a pre-determined distance
toward the brake drum.
As the car moves rearward and the brakes are
applied friction between the primary shoe and the drum forces
the primary shoe against the anchor pin. Hydraulic pressure in
the wheel cylinder forces the
upper end of the secondary shoe away from the anchor pin. As
the secondary shoe moves away from the anchor pin, the upper
end of the adjuster lever is prevented from moving by the actuating
link. This causes the adjuster lever to pivot on the secondary
shoe forcing the adjuster lever against the adjusting screw sprocket.
If the brake linings are worn enough to allow the secondary shoe
to move the pre-determined distance, the adjuster lever will
turn the adjusting screw sprocket one or two teeth, depending
on lining. wear. If the secondary shoe does not move the pre-determined
distance, movement of the adjuster lever will not be great enough
to rotate the adjusting screw sprocket.
When the brakes are released, the adjusting
lever return spring will move the adjuster lever into the adjusting
position on the sprocket.
An override feature is built into the self-adjusting
brake which allows the secondary shoe to be applied in reverse
in the event the adjusting screw becomes "frozen" preventing
the self-adjuster from operating.
When the car is moving forward and the brakes
are applied, the upper end of the secondary shoe is forced against
the anchor pin due to the selfenergizing action of the brakes,
and the selfadjuster does not operate.
