5. • All modern hydraulic wheel brakes of the drum
type have a 'self-energizing" or 'servo' feature in
which the force of the rotating drum is utilized to
increase the brake pressure.
• In Figure when the vehicle is traveling forward, the
drum is rotating in a counterclockwise direction.
• When the brakes are applied, the primary shoe at
the left tends to move in the direction of the drum
rotation, because of the friction of the rotating
drum.
working
6. • Since the primary shoe is linked to the secondary
shoe at the bottom the secondary shoe is forced
around against the anchor pin at the top.
• The result of this wrapping action is that both shoes
are forced into tighter contact with the drum and the
braking pressure is more uniformly applied.
• When the brakes are applied while the car is in
reverse, the secondary shoe tends to move in a
clockwise direction against the primary shoe, forcing
the latter against the anchor pin.
