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The working parts of a drum brake are contained in a hard metal drum
that is attached to the hub of a wheel and revolves with it. Inside,
but unattached to the drum, are a pair of stationary curved brake
shoes that are normally held away from the drum by springs. When the
brake pedal is depressed, fluid is forced through the brake lines and
into the wheel cylinder. Pushrods in the cylinder then apply pressure
to both shoes, overcoming the spring tension and pressing the shoes
against the drum. Hydraulic drum brakes can also be mechanically
activated as parking brakes by a cable attached to the lever. When
pressure is removed from the brake pedal, springs on the brake shoes
force the shoes back to their normal released position. This movement
of the shoes forces the pistons inward, returning the fluid to the
master cylinder reservoir.
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