Chevrolet Light Duty Truck 1973 Service Manual page 271

is pushed forward. The fluid in the master cylinder, and
the entire system, being incompressible, transmits the
force exerted by the master cylinder piston to all the
inner surfaces of the system. At this point only the
pistons in the wheel cylinders or caliper are free to move,
and since the hydraulic fluid is not compressible, the
pistons move outward to force brake shoes against the
brake drums or disc.
To work properly a hydraulic brake system must be
leakproof, sturdy and filled with the right type of fluid.
The right type of fluid is one that has no corrosive effect
on the system's parts, and does not readily freeze, boil or
vaporize at the temperature extremes encountered in
vehicle use.
PRESSURE AND FORCE (Fig. 5)
One of the advantages of a hydraulic system is that we
have the same pressure inside the system and up to all
wheels. This does not mean however that we are
applying the same force at each wheel; in fact, it permits
us to apply unequal force under certain circumstances.
NOTE: Pressure may be different at front or
rear wheels due to valving. Valves are
covered later in this section.
Pressure can be defined as the amount of force applied
to a specific area, (measured in square inches). Suppose a
hydraulic pressure of
10
applied to an object with a surface area of 16 square
inches. The total applied force would equal 160 pounds -
(ten psi pressure times an area of 16 square inches). If
the same psi were applied to an object with a surface
10
area of two square inches, a force of
applied.
Reduced to a formula, this means that pressure (in
pounds per square inch) times area (in square inches)
equals total force applied.
PRESSURE GAUGE
AREA—16 sq. in.
160
POUNDS
10 x 1 6 Sq. in. = 160 lbs.
(psi) x (AREA)= TOTAL FORCE
Fig. 5 -Pressure and Force
psi (pounds square inch) were
pounds would be
20
10 psi
AREA—2 sq. in.
10 x 2 sq. in. = 20 lbs.
(psi) x (AREA) = TOTAL FORCE
This is the principle that enables us to incorporate a very
desirable feature in the brake system - we can vary the
braking power to the wheels by changing the wheel
cylinder piston area.
Some of the vehicle weight is transferred to the front
wheels when the brakes are applied. We can balance this
weight transfer with different braking force at the front
and rear wheels.
BRAKE SYSTEM COMPONENTS
MASTER CYLINDER (Fig. 6)
The master cylinders used today have split reservoirs.
This means that the front and rear brakes are separated
from each other by the design of the master cylinder.
The master cylinder contains two fluid reservoirs and two
cylindrical pressure chambers in which force, applied to
the brake pedal, is transmitted to the fluid which actuates
the brake shoes. Breather ports and compensating ports
permit passage of fluid between each pressure chamber
and its fluid reservoir during certain operating
conditions. A vented cover and flexible rubber
diaphragm, at the top of the master cylinder reservoir,
seal the hydraulic system from possible entrance of
contamination while at the same time permitting
expansion or contraction of fluid within the reservoirs
without direct venting.
In the pressure chambers, coil springs hold rubber
primary seals against the end of the pistons. These seals
and rubber secondary seals on the opposite end of the
pistons, prevent escape of fluid past the pistons. The
piston is retained in the cylinder by a push rod retainer.
A rubber boot is installed over this end of the cylinder to
exclude foreign matter.
Stroking or pushing the brake pedal causes the primary
piston in the master cylinder to move forward. At the
same time, a combination of hydraulic pressure and
force of the primary piston spring moves the secondary
piston forward. When the pistons have moved forward
so that their primary seals cover the compensating ports,
hydraulic pressure is built up and transmitted to the
front and rear wheels.
When the brake pedal is released, the master cylinder
pistons move rearward and hydraulic pressure on the
brake system is released.
Rear Line Failure (Fig. 7)
In case of a ruptured rear brake line or some other
malfunction, the primary piston will move forward when
the brakes are applied, but will not generate hydraulic
pressure. Only a negligible force is transferred to the
secondary piston through the primary piston spring until
the primary piston comes in contact with the secondary
piston. Then, push rod force is transmitted directly to the
secondary piston and sufficient pressure is generated to
operate the front brakes.
BRAKES 5-5
LIGHT DUTY TRUCK SERVICE MANUAL