Chapter 57 Brake Fundamentals
1. Explain the Basic Principles of Braking
Including Friction, pressure, and heat dissipation
A. Introduction (Basic Principles of Braking)
Friction is the force that resist movement between any two contacting surfaces
When a car is traveling at freeway speed
A large amount of kinetic energy is stored
This is energy that wants to remain in motion (inertia)
Energy cannot be consumed, but its form can be changed
When you apply the brakes to stop the car
Dry friction is used to change energy of motion (kinetic energy) to heat energy
The amount of horsepower changed to heat during stopping
Can amount to several times the power developed by the engine during acceleration
The amount of kinetic energy to be changed into heat depends
On the weight and speed of the vehicle
The temperature in brake linings during a stop can approach 600 degrees Fahren height
The amount of dry friction varies, depending on the force applied
The material that the friction surfaces are made of
And the roughness or finish of the friction surface
Coefficient Friction- the ratio of force holding two surfaces
in contact to the force required to slide one over the other
B. Weight Transfer
During a stop the weight of a vehicle
shifts onto the front brakes
Because of this weight transfer rear brakes do not
Usually wear out as fast as the front brakes
Front brakes must be able to absorb more heat than rear brakes
So, linings with more surface are required
Heavier vehicles require wider linings
with more surface area to carry off the increased heat
The ratio between the front and rear brakes is about 60/40
This is for rear-wheel-drive vehicles
When the vehicle has front wheel drive
However, the braking ratio is about 80 percent for the front
And the rear is about 20 percent
This is because of the added weight of the powertrain components in the front
C. Brake Linings
The friction materials used in cars and trucks are called brake linings
On drum brakes linings are called shoes
On disc brakes they are called pads
Linings are either bonded (glued) or riveted to the shoe
(on drum brakes) or disc backing
Some newer pads are integrally molded
From the back of the pad, you observe holes
That are either full or partially full of lining material
The linings are molded to the metal disc
backing plate to become one unit
Linings are asbestos, nonmetallic organic, semimetallic, metallic, or ceramic
Asbestos is hazardous and have been phased out of brake material
Semimetallic linings are organic
with sponge iron and steel fibers mixed into them
Which add and temperature resistance
They are fast at taking heat away from the rotor and putting it into the lining
This heat transfer does not affect the service life of the lining
Metallic linings are used in heavy duty racing conditions
They work poorly when cold
Ceramic linings are original equipment
on about half of all new vehicles
They use ceramic and copper fibers to control heat
The ceramic material dampens some noise
They produce lighter color brake dust
D. Drum and disc Brakes
There are two main types of brakes
Drum and disc
Late model vehicles use disc brakes in the front and drum in the back
Drum brake systems have metal brake drums
that are bolted to the wheels
Brake drums are made of several materials and styles
The linings and braking components are mounted on a fixed backing plate
Disc brake systems have a rotor and caliper
Similar to a bicycle brake
2. Describe Hydraulic System Operation
Including master cylinder, control valves, and safety switches
A. Hydraulic System Operation
When a driver depresses the brake pedal
The linkage applies force to the piston at the rear of the master cylinder
The master cylinder pressurizes the brake fluid
It sends it to the wheel cylinders and the calipers
Since 1967 all master cylinders have two chambers
B. Master Cylinder Operation
The master cylinder is connected to the foot pedal
It supplies hydraulic pressure
This is to operate the wheel cylinders during braking
The primary cup compresses fluid when the pedal is depressed
A secondary seal prevents fluid from leaking
on the back of the master cylinder bore
This seal does not seal against pressure
The master cylinder bore is filled with fluid from the reservoir
It comes from the reservoir through the vent port
This allows for expansion of the fluid as it absorbs heat
When the pedal is applied
The primary cup moves forward in the master cylinder bore
It then goes past the vent port
With the vent port isolated
Fluid movement is restricted
As the pedal is forced further toward the floor
Fluid pressure builds in the hydraulic system
C. Seal Lips are Directional
In a simple drum brake system
The lips of the cup seal face inward
against the pressurized fluid created by the master cylinder
The lip on the master cylinder primary cup
Faces toward the fluid
The wheel cylinder has a pair of rubber cup lips seals
They face in an opposing direction to master cylinder fluid pressure
A seal installed in this direction will seal
However, a seal installed backwards will leak
D. Tandem Master Cylinder
Since 1967 tandem master cylinders
With two separate hydraulic systems
Have been mandated by law on all cars
A tandem master cylinder has one cylinder bore
With two separate pistons and chambers
During normal stopping, the primary cup
On the rear system pushes fluid to the brakes it serves
It also pushes fluid forward against a rearward facing lip seal on the front piston
When the friction materials begin to apply force at the wheels
Pressure begins to build in both the front and rear systems at the same time
When one half of a tandem system fails
The pedal will be lower but the remaining half of the hydraulic system
Should have enough braking capacity to stop the car
During a failure in part of the brakes served by the rear piston
A stub on the front of that piston bottoms out against the back of the front piston
Thus, the primary piston is applied mechanically instead of hydraulically
E. Master Cylinder Reservoir
Most master cylinders today are aluminum
With a plastic reservoir sealed to the cylinder with rubber grommets
They are called composite master cylinders
They are made of two materials
Aluminum is lighter than cast iron and less expensive to manufacture
Plastic reservoirs are transparent
This allows the fluid level to be observed without removing the reservoir cover
Which could lead to dirt or moisture contamination
Master cylinder fluid level fluctuates through the vent port as the brakes are used
Because the fluid heats up or cools down
The cover of the master cylinder must include
a feature that prevents vacuum lock
as the expanded brake fluid cools down
If a vacuum lock were to occur
Air would be drawn into the brake fluid past the secondary cup
At the rear of the master cylinder
Master cylinder reservoirs are prevented of vacuum locking in one of two ways
There is either flexible rubbing diaphragm inside the cover
Or a screw cap, or there is a plastic float
These devices allow atmospheric pressure
to act on the fluid in the reservoir without the fluid becoming contaminated
with moisture from the outside air
E. Pressure Differential Switch/Brake Warning Light
Tandem switches are equipped with a brake warning light
It alerts the driver when half of the hydraulic system has failed
When pressure on one side of the system drops
A piston inside the hydraulic safety switch moves off-center
This completes an electrical circuit
Which illuminates a warning lamp
on the drivers compartment instrument panel
On some cars, the lamp illuminates
when the parking brake is applied
This system also illuminates the brake warning light
Some vehicles have wear sensors in the disc brake pads
They turn on the same lamp when the pads wear beyond their limit
D. Hydraulic Control Valves
Additional hydraulic control valves were required
Mostly on older vehicles with hybrid (disc/drum) brake systems
Called metering and proportional valves
They were required to balance braking force
Between the front and rear brakes
E. Metering Valve
A metering valve is used on front disc brakes
When the car has rear drum brakes
It prevents the front brakes from applying
until the rear drum brake shoes overcome return spring pressure
and contact the brake drums
The metering valve does not operate
until system pressure reaches about 10 psi
Keeping the valve open below this pressure allows the fluid
To expand and contract normally when the brakes are not applied
When the pressure builds up to 10 psi
The metering valve shuts off pressure to front brakes
Until the rear brake hydraulic system has developed pressure of 500 to 300 psi
When the metering valve operates
the stem can be seen moving out
Away from the body of the valve
The movement of the valve can be felt slightly at the brake pedal
During very light brake pedal application
F. Proportioning Valve
More force is required to apply disc brakes than drum brakes
During hard stopping weight shifts forward
and away from the rear wheels
The ability of brakes to do their jobs
is limited by the grip of the tires to road surface
Remember that dual-servo drum brakes are self-energized
Which causes rear wheel lockup to be a problem on some vehicles
Older vehicles used proportioning valves to prevent this
Newer vehicles control wheel lockup using ABS
3. Understand the Operation of Power Brakes
A. Power Brakes
Power brakes reduce brake pedal effort
However, they allow reasonable pedal feedback
This allows the driver to have a feel during braking
Disc brakes have been common on the fronts of vehicles
This has been since the 1970s
They have the ability to apply more stopping energy at the wheels
They also require more application effort
However, they don’t self-energize like drum brakes
Gripping the smooth shiny rotor between two relatively smooth brake pads
Requires a substantial increase in braking effort
As a result, power brakes have been installed on most vehicles
Since the late 1970s
For modern brake systems
The most effective and economical means of boosting brake pressure
Is to use a vacuum assist brake device called a booster
This is installed behind the master cylinder
A brake booster often allows the master cylinder to have a larger bore
This allows the brakes to apply with less pedal travel
A large-bore master cylinder will exert
less pressure on the brake fluid
However, will move a larger volume of fluid
in the same amount of pedal movement
A vacuum power booster mounted behind the master cylinder
Is the type used on most vehicles
It uses the difference between atmospheric pressure and vacuum
This is to apply force
The brake booster is powered by vacuum
It is supplied from the engine's intake manifold
The power brake booster and master cylinder
Are mounted on the bulkhead (firewall)
This is the metal wall between the engine and passenger compartments
The brake booster increases the braking effort
Which is supplied by the driver
If the brake booster fails
The service brakes will still operate
Although more pedal pressure will be required
B. Brake Booster Check Valve
Most power brake systems depend on engine vacuum
If the engine stalls the power brake will still work
Federal standards call for at least one power assisted stop
This is if the engine dies, and power is lost
The second stop will have less assist than the first stop
The third stop will have less assist than the second stop
And so, until there is no further assist
The power brake booster provides reserve braking
This is because it has a check valve
It is usually located in the front brake booster at the end of the hose
It supplies vacuum from the intake manifold
The check valve is a one-way valve
It prevents manifold vacuum from leaving the power booster
If engine vacuum drops below the tension of its spring
When vacuum drops or the engine is shut off
The check valve moves against its seat
It maintains a supply of vacuum in the power booster reservoir
And it prevents air from entering
C. Vacuum Booster Operation
The most common type of power brake booster is
The vacuum suspended power brake
The booster is a metal chamber divided by a rubber diaphragm
Each side is isolated by the control valve
It is located at the back side of the power booster
At the end of the brake pedal apply rod
When the brakes are released, and the engine is running
There is a vacuum on both sides of the booster diaphragm
The control valve directs air movement in the power booster
Resulting in an assist to pedal pressure when needed
C. Other Power Brake Types
Other power brakes include
Hydraulic power assist, and electric power assist
Some power brake systems use hydraulic fluid pressure
This is used instead of vacuum to produce power assist
A power brake system powered off the power steering pump
Is called a hydro-boost brake
The power booster is located
In the same place as the conventional vacuum power booster
It fits into a similar amount of area
Hydro-boost systems are found on some turbo charged gasoline engines
They have problems with vacuum supply
This is because they have pressure in the intake manifold
when the turbocharger is working
Hydro-boost is also found on vehicles with diesel engines
Diesel engines do not have a throttle plate
They do not produce sufficient vacuum to operate a power booster