TSE DP20-4H, Owner'S Manual

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Hydraulic Brake Operation
Duo-Servo
The duo-servo brake uses a dual piston wheel cylinder to apply
the brakes. This type of brake is typically used in a vacuum/
hydraulic, electric/hydraulic or air/hydraulic system. A description
of operation of this brake is as follows:
When the brakes are applied, the double-acting wheel cylinder
moves the primary and secondary shoes towards the drum. The
frictional force between
the brake drum and lining
attempts to turn the
primary shoe into the
secondary shoe. The
secondary shoe is forced
onto the anchor pin and
from this point, the
secondary and primary
shoes attempt to "wrap
around". In essence, the
brake has utilized frictional
force to help the applying force on both shoes.
If the brakes are applied while the vehicle is backing, the shoes
rotate in the direction of the drum rotation. This causes the
secondary shoe to leave the anchor and causes the primary shoe
to move against the anchor. Action of the brake is the same in
reverse as forward.
Uni-Servo
This type of hydraulic brake utilizes a single acting cylinder. Upon
actuation, the primary shoe is pressed against the brake drum,
which causes the shoe to move in the direction of rotation. This
movement in turn actuates the secondary shoe through the
adjuster assembly. Braking in reverse is significantly less
effective than in the forward direction.
Another variation is called a “free backing” brake which is
commonly used on trailers with a surge hitch system. When
Braking Systems - Hydraulic
Anchor Post
Retractor
Springs
Backing
Plate
Secondary
Shoe
Adjuster Spring
Adjuster Assembly
Primary Shoe
Hold Down
Spring
Actuating Pin
Hydraulic Wheel
Cylinder