chap69 pps Automotive technology at University of Cambridge

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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OBJECTIVES:
After studying Chapter 69, the reader should
be able to:

•
•
•

Prepare for ASE Brakes (A5) certification test content area
“A” (Hydraulic System Diagnosis and Repair).
State Pascal’s Law.

Describe the function, purpose, and operation of the master
cylinder.

Continued
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OBJECTIVES:
After studying Chapter 69, the reader should
be able to:

•
•
•

Explain how hydraulic force can be used to supply high
pressures to each individual wheel brake.
Describe the process of troubleshooting master cylinders and
related brake hydraulic components.
Explain how a quick take-up master cylinder works.

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KEY TERMS:
bore master cylinder • breather port • bypass port • bypassing
compensating port
diagonal split master cylinder • dual split master cylinder

fast-fill master cylinder • filler port
hydraulic system • master cylinder
Pascal’s Law • pedal free play • pedal height • pedal reserve
distance • piston assemblies
Continued
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KEY TERMS:
quick take-up master cylinder
replenishing port
self-apply • step-bore master cylinder
vent port

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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HYDRAULIC PRINCIPLES
In addition to mechanical advantage provided by leverage, all
vehicles use hydraulic pressure to increase brake application force.
All braking systems require that a driver’s force is transmitted to
the drum or rotor attached to each wheel. The force exerted on the
brake pedal varies due to strength and size of the driver.
Figure 69–1 Hydraulic brake lines transfer the
brake effort to each brake assembly attached
to all four wheels.

Engineers design brake systems

to require less than 150 lb of
force (68 kg) from the driver,
yet provide the force necessary
to stop a heavy vehicle from
high speed.
Continued

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NONCOMPRESSIBILITY OF LIQUIDS
Hydraulic systems use liquids to transmit motion. For all
practical purposes, a liquid cannot be compressed.
No matter how much pressure or

force is placed on a quantity of
liquid, its volume will remain
the same.
Figure 69–2 Because liquids cannot be
compressed, they are able to transmit
motion in a closed system.

This fact enables liquids in a
closed system to transmit
motion.
Continued
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Liquids cannot be compressed, but any air trapped in the system
can be compressed. The hydraulic system is air-contaminated.
Even though piston A is moved 1 in., piston B will not move if the
load on it is greater than the pressure of the air in the system.
Figure 69–3 Hydraulic systems must be free
of air to operate properly.

If the load on piston B is 50
pounds per square inch (psi),
the movement of piston A
must compress the air in the
system to that same pressure
before piston B will move.
A brake system must be air
free or there will be serious
problems.

Continued

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PASCAL’S LAW
The hydraulic principles that permit a brake system to function were discovered by a French physicist,
Blaise Pascal (1632–1662). Pascal’s Law states that “when force is applied to a liquid confined in a
container or an enclosure, the pressure is transmitted equal and undiminished in every direction.”
Assume a force of 10 lb is exerted on a piston with a surface area of 1 square inch (sq. in.). Since this force
measured in lb or Newtons (N) is applied to a piston with an area measured in square inches (sq. in.), the
pressure is the force divided by the area or “10 pounds per square inch” (psi).
It is this “pressure” that is transmitted, without loss, throughout the entire hydraulic system. See Figure 69–
4.

Continued
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Figure 69–4 A one-pound force exerted on a small piston in a sealed system transfers the
pressure to each square inch throughout the system. In this example, the 1-lb force is able to
lift a 100-lb weight because it is supported by a piston that is 100 times larger in area than the
small piston.

Continued
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If you know two out of the three factors, you can calculate the
other using this formula:

See the formula on Page 826 of your textbook.
A practical example involves a master cylinder with a piston area of
1 sq. in., and one wheel cylinder with an area of 1 sq. in., and one
wheel cylinder with a piston area of 2 sq. in. See Figure 69–5.
Continued
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Figure 69–5
The amount of force on the piston is the result of pressure multiplied by the surface area.

Continued
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The real “magic” of a hydraulic brake system is the fact that
different forces can be created at different wheel cylinders. More
force is necessary for front brakes than for rear brakes because, as
the brakes are applied, the weight of the vehicle moves forward.
Larger (area) pistons are used in wheel cylinders (calipers, if disc
brakes) on front wheels to increase force to apply front brakes.
Not only can hydraulics act as a “force machine” (by varying

piston size), but the hydraulic system also can be varied to change
piston stroke distances.
On a typical vehicle, a driver-input force of 150 lb (660 Newtons)
is boosted both mechanically (through the brake pedal linkage)
and by the power booster to a fluid pressure of about 1700 psi
(11,700 kPa).

Continued

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NOTE: During a typical brake application, only about 1 teaspoon (5 ml
or cc) of brake fluid actually is moved from the master cylinder and into

the hydraulic system to cause the pressure buildup to occur.

With a drum brake, the wheel cylinder expands and pushes the brake
shoes against a brake drum.

The distance the shoes move is
about 0.005–0.012 in. (5 to 12
thousandths of an inch) (0.015–
0.30 mm).
Figure 69–6 Drum brake illustrating
the typical clearance between the
brake shoes (friction material) and
the rotating brake drum represented
as the outermost black circle.
Continued
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With a disc brake, brake fluid pressure pushes on the piston in the
caliper a small amount and causes a clamping of the disc brake pads
against both sides of a rotor (disc).
Figure 69–7 The brake pad (friction material) is
pressed on both sides of the rotating rotor by
the hydraulic pressure of the caliper.

The typical distance the pads
move is only about 0.001–0.003
in. (1-to 3-thousandths-of an
inch) (0.025–0.076 mm).

Continued
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Hydraulic Pressure and Piston Size
If a mechanical force of 100 lb is
exerted by the brake pedal pushrod
onto a master cylinder piston with 1
sq. in. of surface area, the equation
reads:

The result in this case is 100
psi of brake system hydraulic
pressure.
Figure 69–8 Mechanical force and the master
cylinder piston area determine the hydraulic
pressure in the brake system.
Continued
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However, if the same 100-lb force is applied to a master cylinder
piston with twice the area (2 sq. in.), the equation will read:

Doubling the area of the master cylinder piston cuts the hydraulic
system pressure in half. Conversely, if the same 100-lb force is
applied to a master cylinder piston with only half the area (0.5 or
1/2 sq. in.), the equation will show the system pressure is doubled:

Continued
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Application Force and Piston Size
While size of the master cylinder
piston affects hydraulic pressure of
the entire brake system, weight shift
and bias require the heavily loaded
front brakes receive much higher
application force than the lightly
loaded rear brakes.
Pascal states that pressurized liquid
in confined space acts with equal
pressure on equal areas.
100 psi from the master cylinder
will result in 100 psi of friction
assembly application force.
Continued
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Figure 69–10 Differences in brake
caliper and wheel cylinder piston
area have a major effect on brake
application force.

It is piston surface area, not
diameter, that affects force.

In the simple brake system,
the pedal and linkage apply
a 100-lb force on a master
cylinder piston with an area
of 1 sq. in.
This results in a pressure of
100 psi throughout the
hydraulic system.
Continued
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Piston Size versus Piston Travel In disc brakes, mechanical
force available to apply the brakes is four times greater because
of size differences between master cylinder and caliper pistons.

Some hydraulic energy is converted into increased mechanical
force. The tradeoff is the larger caliper piston with greater force
will not move as far as the smaller master cylinder piston.
Hydraulic energy converted into mechanical motion is decreased.

The relative movement of pistons
within the brake system can be
calculated with the following
equation:

Continued
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Figure 69–11 The increase in application force created by the large brake caliper piston is
offset by a decrease in piston travel.

The results show that
if the master cylinder
piston stroke is 1 in.,
the caliper piston will
move only 1/4 inch.

Continued
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Figure 69–12 The decrease in application force created by a small wheel cylinder piston is
offset by an increase in piston travel.

If caliper piston area were
reduced to 2 sq. in.,
application force would
increase to only 200 lb,
the caliper piston would
travel 1/2 inch for a 1 in.
master cylinder stroke.

With a dual- piston
cylinder, the total travel
is divided between the
two pistons.
Continued
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Hydraulic Principles and Brake Design When a brake system
is designed, the hydraulic relationships discussed above play a
major part in determining the sizes of pistons within the system.
The piston sizes selected must move enough fluid to operate the
wheel cylinder and brake caliper pistons through a wide range of

travel, while at the same time they must create enough application
force to lock the wheel friction assemblies.

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MASTER CYLINDERS
The master cylinder is the heart of the entire braking system. No
braking occurs until the driver depresses the brake pedal. The brake
pedal linkage applies the force of the driver’s foot into a closed
hydraulic system.
Master Cylinder Reservoirs Most vehicles built since the early
1980s are equipped with see-through master cylinder reservoirs,
which permit owners and service technicians to check the brake

fluid level without having to remove the top of the reservoir.
Some countries have laws that require this type of reservoir.
See Figure 69–13.
Continued
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Figure 69–13 Typical master cylinder showing the
reservoir and associated parts. The reservoir
diaphragm lays directly on top of the brake fluid,
which helps keep air from the surface of the brake
fluid because brake fluid easily absorbs moisture
from the air.

Reservoir capacity is great enough
to allow for the brakes to become
completely worn out and still have
enough reserve for safe operation.
The typical capacity of the entire
braking system is usually 2 to 3
pints (1 to 1.5 liters).
Vehicles equipped with four-wheel
disc brakes usually hold 4 pints (2
liters) or more.
Continued
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