Manual Drivetrains and Axles Fourth Edition

1
(No Transcript)
2
OBJECTIVESAfter studying Chapter 7, the reader
should be able to1. Prepare for the Brakes
(A5) ASE certification test content area A
(Hydraulic System Diagnosis and Repair).2.
State Pascals law.3. Describe the function,
purpose, and operation of the master cylinder.4.
Explain how hydraulic force can be used to
supply high pressures to each individual wheel
brake.5. Describe the process of
troubleshooting master cylinders and related
brake hydraulic components.6. Explain how a
quick take-up master cylinder works.
3
HYDRAULIC PRINCIPLESIn addition to the
mechanical advantage provided by leverage, all
vehicles use hydraulic pressure to help increase
brake application force.
4
All braking systems require that a driver's force
is transmitted to the drum or rotor attached to
each wheel.
5
Engineers design braking 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.
6
Noncompressibility of LiquidsHydraulic 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.
7
If piston A is moved a distance of 1 inch, the
liquid will be displaced ahead of it and piston B
will move 1 inch as well.
8
Liquids cannot be compressed, but any air trapped
in the system can be compressed. The simple
hydraulic system has been contaminated with air.
9
A brake hydraulic system must be air free or
there will be serious problems.
10
PASCAL'S LAWThe hydraulic principles that
permit a brake system to function were discovered
by a French physicist, Blaise Pascal (1632-1662).
Pascals 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. Since this
force measured in pounds (lb.) or Newtons (N) is
applied to a piston with an area measured in
square inches, 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.
11
(No Transcript)
12
F force (lb.) (Newtons)P pressure in lb.
per sq. in. (kilo Pascals kPa)A area in sq.
in. (cm2)F P ? A (force is equal to the
pressure multiplied by the area)P F ?
A (pressure is equal to the force divided by the
area)A F ? P (area is equal to the force
divided by the pressure)
13
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.
14
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
build-up to occur.
15
With a drum brake, the wheel cylinder expands and
pushes the brake shoes against a brake drum. The
distance the shoes move is only about 0.005-0.012
in. (5 to 12 thousandths of an inch) (0.015-0.30
mm).
16
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).
17
Hydraulic Pressure and Piston SizeIf a
mechanical force of 100 pounds is exerted by the
brake pedal pushrod onto a master cylinder piston
with 1 square inch of surface area, the equation
reads as follows 100 pounds 100
psi1 sq. in.The result in this case is 100 psi
of brake system hydraulic pressure.
18
(No Transcript)
19
Application Force and Piston SizePascals law
states that a pressurized liquid in a confined
space acts with equal pressure on equal areas and
as long as the pistons in a hydraulic system have
the same area, 100 psi from the master cylinder
will result in 100 psi of friction assembly
application force.
20
(No Transcript)
21
In the simple brake system, the pedal and linkage
apply a 100-pound force on a master cylinder
piston with an area of 1 square inch.
22
Piston Size Versus Piston TravelIf the master
cylinder piston stroke is 1 inch, the caliper
piston will move only 1/4 inch.
23
With a dual-piston wheel cylinder, the total
travel is divided between the two pistons.
24
Hydraulic Principles and Brake DesignThe 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.
25
A technician should never change the sizing of
any hydraulic brake component on any vehicle!
26
MASTER CYLINDERSThe master cylinder is the
heart of the entire braking system. No braking
occurs until the driver depresses the brake
pedal. The brake pedal linkage is used to apply
the force of the driver's foot into a closed
hydraulic system.
27
Master Cylinder Reservoirs
28
The 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.
29
Master Cylinder Reservoir DiaphragmThe
reservoir is vented to the atmosphere so the
fluid can expand and contract without difficulty
as would be the case if the reservoir were sealed.
30
Whenever servicing a brake system, be sure to
check that the vent hole is clear on the cover to
allow air to get between the cover and the
diaphragm.
31
There are two reasons why the customer should be
told not to fill the master cylinder reservoir
when the brake fluid is down to the "minimum"
mark.
32
1. If the master cylinder reservoir is low,
there may be a leak that should be repaired.2.
As the brakes wear, the disc brake piston moves
outward to maintain the same distance between
friction materials and the rotor. Therefore, as
the disc brake pads wear, the brake fluid level
goes down to compensate.
33
MASTER CYLINDER OPERATIONThe master cylinder is
the heart of any hydraulic braking system. Brake
pedal movement and force are transferred to the
brake fluid and directed to wheel cylinders or
calipers.
34
(No Transcript)
35
The master cylinder is also separated into two
pressure-building chambers (or circuits) to
provide braking force to one-half of the brake in
the event of a leak or damage to one circuit.
36
Both pressure-building sections of the master
cylinder contain two holes from the reservoir.
The Society of Automotive Engineers (SAE) term
for the forward (tapered) hole is the vent port,
and the rearward straight drilled hole is called
the replenishing port.
37
The vent port is also called the compensating
port.The replenishing port is the low-pressure
rearward, larger diameter hole. The inlet port
is also called the bypass port, filler port, or
breather port.
38
At-Rest Position The primary sealing cups are
between the compensating port hole and the inlet
port hole. In this position, the brake fluid is
free to expand and move from the calipers, wheel
cylinders, and brake lines up into the reservoir
through the vent port (compensation port) if the
temperature rises and the fluid expands. If the
fluid was trapped, the pressure of the brake
fluid would increase with temperature, causing
the brakes to selfapply.
39
(No Transcript)
40
Applied PositionWhen the brake pedal is
depressed, the pedal linkage forces the push rod
and primary piston down the bore of the master
cylinder.
41
As the piston moves forward, the primary sealing
cup covers and blocks off the vent port
(compensating port). Hydraulic pressure builds
in front of the primary seal as the pushrod moves
forward. The back of the piston is kept filled
through the replenishing port.
42
Released PositionReleasing the brake pedal
removes the pressure on the pushrod and master
cylinder pistons. As the piston is pushed back,
the lips of the seal fold forward allowing fluid
to quickly move past the piston.
43
Always Check for Venting (Compensation)Remove
the reservoir cover and observe the brake fluid
for spurting while an assistant depresses the
brake pedal.
44
Normal Operation (movement of fluid observed in
the reservoir) There should be a squirt or
movement of brake fluid out of the vent port of
both the primary and secondary chambers.
45
No movement of fluid observed in the reservoir in
the primary piston This can be caused by the
following.a. Incorrect brake pedal height.b.
A defective or swollen rubber sealing cup on the
primary piston could cause the cup itself to
block the vent port.
46
DUAL-SPLIT MASTER CYLINDERSDual-split master
cylinders use two separate pressure-building
sections. One section operates the front brakes
and the other section operates the rear brakes on
vehicles equipped with a front/rear-split system.
47
The nose end of the master cylinder is the
closed end toward the front of the vehicle. The
open end is often called the pushrod end of the
master cylinder.
48
To permit the operation of the secondary piston
(nose end piston) in the event of a hydraulic
failure of the rear section, the primary piston
extension will mechanically contact and push on
the secondary piston.
49
(No Transcript)
50
DIAGONAL-SPLIT MASTER CYLINDERSWith front-wheel
drive vehicles, the weight of the entire power
train is on the front wheels and 80 to 90 of the
braking force is achieved by the front brakes.
If the front brakes fail, the rear brakes alone
would not provide adequate braking force. The
solution is the use of a diagonal-split master
cylinder.
51
(No Transcript)
52
(No Transcript)
53
In a diagonal-split braking system, the left
front brake and the right rear brake are on one
circuit, and the right front with the left rear
is another circuit of the master cylinder.
54
QUICK TAKE-UP MASTER CYLINDERSMany newer
vehicles use low drag disc brake calipers to
increase fuel economy.
55
The design of a quick take-up master cylinder
includes a larger diameter primary piston
(low-pressure chamber) and a quick take-up valve.
This type of master cylinder is also called
dual-diameter bore, step-bore, or fast-fill
master cylinders.
56
A spring-loaded check ball valve holds pressure
on the brake fluid in the large-diameter rear
chamber of the primary piston. When the brakes
are first applied, the movement of the rear
larger piston forces this larger volume of brake
fluid forward past the primary piston seal and
into the primary high-pressure chamber.
57
At 70 to 100 psi, the check ball valve in the
quick take-up valve allows fluid to return to the
brake fluid reservoir. Because the quick take-up
"works" until 100 psi is reached, a metering
valve is not required to hold back the fluid
pressure to the front brakes.
58
(No Transcript)
59
(No Transcript)
60
DIAGNOSING AND TROUBLESHOOTING MASTER
CYLINDERSA thorough is important when
inspecting any master cylinder. The visual
inspection should include checking the following
items1. Check the brake fluid for proper
level and condition.2. Check that the vent
holes in the reservoir cover are open and
clean.3. Check that the reservoir cover
diaphragm is not torn or enlarged.4. Check for
any external leaks at the lines or at the pushrod
area.
61
After a thorough visual inspection, check for
proper operation of pedal height, pedal free
play, pedal reserve distance, and pedal travel.
62
(No Transcript)
63
(No Transcript)
64
(No Transcript)
65
The Brake Pedal Depressor TrickThe master
cylinder can be used to block the flow of brake
fluid.To prevent brake fluid loss that can
easily empty the master cylinder reservoir,
simply depress the brake pedal slightly or prop a
stick or other pedal depressor to keep the brake
pedal down. The master cylinder stays full and
the brake fluid stops dripping out of brake lines
that have been disconnected.
66
(No Transcript)
67
Spongy Brake PedalA spongy pedal with a larger
than normal travel indicates air in the lines.
68
Lower Than Normal Brake PedalA brake pedal that
travels downward more than normal and then gets
firm is an indication that one circuit of the
dual-circuit hydraulic system is probably not
working. A lower than normal brake pedal may
also be an indication of air in the hydraulic
system.
69
Sinking Brake Pedal If the brake pedal sinks
all the way to the floor, suspect a defective
master cylinder that is leaking internally. This
internal leakage is often called bypassing
because the brake fluid is leaking past the
sealing cup.
70
DISASSEMBLY OF THE MASTER CYLINDERCheck the
vehicle manufacturers recommendation before
attempting to overhaul or service a master
cylinder. Many manufacturers recommend replacing
the master cylinder as an assembly.Step 1.
Remove the master cylinder from the vehicle.
71
Step 2. Remove the reservoir.
72
Step 3. Remove the retaining bolt that holds the
secondary piston assembly in the bore.Step 4.
Depress the primary piston with a blunt tool such
as a Phillips screwdriver, a rounded wooden
dowel, or an engine pushrod.CAUTION If holding
the master cylinder in a vise, use the flange
area. Never clamp the body of the master
cylinder.
73
Remove the snap ring and slowly release the
pressure on the depressing tool. Spring pressure
should push the primary piston out of the
cylinder bore.
74
Step 5. Remove the master cylinder from the vise
and tap the open end of the bore against the top
of a workbench to force the secondary piston out
of the bore. If necessary, use compressed air in
the outlet to force the piston out.CAUTION Use
extreme care when using compressed air. The
piston can be shot out of the master cylinder
with a great force.
75
INSPECTION AND REASSEMBLY OF THE MASTER
CYLINDERInspect the master cylinder bore for
pitting, corrosion, or wear. Most cast-iron
master cylinders cannot be honed because of the
special bearingized surface finish that is
applied to the bore during manufacturing. Slight
corrosion or surface flaws can usually be removed
with a hone or crocus cloth. Otherwise, the
master cylinder should be replaced as an assembly.
76
Aluminum master cylinders have an anodized
surface coating applied that is hard and wear
resistant. Honing would remove this protective
coating.
77
Thoroughly clean the master cylinder and any
other parts to be reused (except for rubber
components) in clean denatured alcohol. If the
bore is okay, replacement piston assemblies can
be installed into the master cylinder after
dipping them into clean brake fluid.
78
Step 1. Install the secondary (smaller) piston
assembly into the bore, spring end first.
79
Step 2. Install the primary piston assembly,
spring end first.Step 3. Depress the primary
piston and install the snap ring.Step 4.
Install the stop bolt.
80
Step 5. Reinstall the plastic reservoir, if
equipped.
81
Step 6. Bench bleed the master cylinder.
82
SUMMARY1. 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.2. Pascals law
states that When a force is applied to a
liquid confined in a container or enclosure, the
pressure is transmitted equally and undiminished
in every direction.3. Master cylinder
reservoirs are large enough for the brakes to be
worn completely down and still have a small
reserve.4. The front port of the master
cylinder is called the compensating ort and the
rear port is called the inlet port.5. Brake
system diagnosis should always start with
checking for venting (compensation).6.
Dual-split master cylinders that separate the
front brakes from the rear brakes are used on
rear-wheel-drive vehicles.7. Diagonal-split
master cylinders that separate right front and
left rear from the left front and right rear
brakes are used on front-wheel-drive vehicles.8.
Some master cylinders can be rebuilt, but the
cylinder bore should not be honed unless
recommended by the manufacturer.
83
REVIEW QUESTIONS1. Explain Pascals law.2.
Describe how a master cylinder works.3. Discuss
the difference between a dual-split and a
diagonal-split master cylinder.4. What is the
difference between checking for venting
(compensation) and bypassing?