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Level 2 Diploma in Light Vehicle Maintenance & Repair Principles (QCF) Unit 154 Handout 6
Unit 154: Knowledge of light vehicle chassis units and
components
Handout 6: Construction and operation of braking systems
The braking system
The purpose of the braking system is to convert the kinetic (moving) energy of the
moving vehicle into heat energy by the use of friction.
The system consists of:
• foot or main braking system
This is usually hydraulically operated and can use disc or drum brakes, or a
combination of both, and will operate on all wheels. Normally, a servo system will
be incorporated to assist the force applied by the driver. Originally single circuit
braking systems were used, but now they are normally dual circuit.
• parking brake system
This is usually mechanical, but may be applied by other means, and operates on
two wheels only. In most cases this is used on the rear wheels, but may be used
on the front wheels. When single circuit brake systems were used, the parking
brake also acted as a secondary braking system.
Drum braking systems
Originally drum brakes were fitted to all four wheels, but in most cases where they are
used now they are only on the rear.
The drum brake uses two brake shoes that are fitted
with a high friction material. When the brakes are
operated, the two shoes are forced out to contact the
drum. As the friction is converted into heat it slows
the vehicle down. To assist with the braking force
the shoes will provide some ‘self-servo action’. The
brake shoes can slide and as they contact the drum
they are forced to move slightly, creating a ‘wedge’
action which will increase the braking force.
• dual servo brake (two leading shoe)
This uses two cylinders to operate the brake shoes.
As the drum rotates in the forward direction, both
shoes will provide a self-servo action, providing a
greater increase in braking force.
When rotating in the opposite direction, this self-
servo action is not produced, so the braking force
when reversing is much less. These are usually
fitted to the front wheels.
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Level 2 Diploma in Light Vehicle Maintenance & Repair Principles (QCF) Unit 154 Handout 6
• leading and trailing brake
This uses one cylinder to operate both brake
shoes. As the drum rotates in a forward
direction, one shoe will provide some self-
servo action. When rotating in the opposite
direction, the other shoe will provide self-servo
action. Although this provides less braking
force, it does provide some self servo-action in
both directions.
When used, these are usually fitted to the rear
wheels.
• adjustment
To provide a constant pedal or handbrake
movement as the brake shoes wear, some
form of adjustment must be provided.
In some cases the brake shoe adjustment is
by manual means, but with most vehicles it is
now automatic. This is usually arranged to
adjust the brake shoes as the movement
exceeds a set limit.
With parking brakes there can also be wear in
the mechanical linkage; this can be manually
or automatically adjusted.
• parking brake
A mechanical system is used to operate the
brake shoes when the parking brake is
applied. This may apply one or both shoes,
depending on the arrangement.
Disc brakes
For many years, the drum brake was the most popular arrangement, but for modern use
it has some disadvantages, the main one being high temperatures during operation
which could lead to a condition known as ‘brake fade’. This is where the friction level
(coefficient of friction) reduces as the temperature increases. Normally, light vehicles will
be fitted with disc brakes at the front and in most cases they will also be fitted at the rear.
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Level 2 Diploma in Light Vehicle Maintenance & Repair Principles (QCF) Unit 154 Handout 6
The disc brake consists of a disc which rotates
with the wheel and a calliper which applies the
brake pads to the disc. Early types were fixed
callipers, where the calliper had two pistons –
one to operate each brake pad – and was
rigidly mounted. Later types use a single piston
which is used to operate both brake pads. The
piston usually operates directly on to one pad
and, as the unit slides, it operates the other
pad.
To assist with cooling, ventilated discs are often
used. The slots between the two sections allow
air to circulate.
• Adjustment
This is an automatic process. When little or no wear has taken place, as the
calliper piston moves it will cause the seal to distort slightly and when the brake
is released the seal returns to its original shape and allows the piston to move
sufficiently to release the brake. As the pad wears, the piston movement is
increased, so when the seal reaches the limit of its natural distortion, the piston
will move through the seal and take up a new position, automatically
compensating for any wear. Some disc brakes do have a manual adjustment, but
this is usually linked to the parking brake.
• Wear indication
As the pads wear, the reduced thickness will cause heat build up and can result
in overheating the brake fluid and damaging the brake disc.
A sensor is fitted into the brake pad; when the
pads wear down to the limit, the sensor is
exposed. As this contacts the disc it completes
an electric circuit and operates a warning device
in the vehicle.
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Level 2 Diploma in Light Vehicle Maintenance & Repair Principles (QCF) Unit 154 Handout 6
• parking brake
A mechanical system can be used to operate the brake pads to provide a parking
brake. In order to operate efficiently, the disc brake requires higher operating
forces and this can make mechanical linkages more complex. One solution has
been to use a drum brake arrangement inside the rear disc brake.
Hydraulic braking systems
A large force is required to slow or stop the vehicle and this is achieved by using a
hydraulic system to increase the force applied by the driver. The main components, and
their basic operation, are:
• master cylinder
This consists mainly of a piston, seals and valves.
As the driver presses the brake pedal, it operates a
cylinder which pressurises the fluid; this then
passes through a valve into the brake circuit . The
valve is used to control the flow and return of the
fluid.
• pipes and hoses
These carry the fluid, under pressure, to the various parts of the circuit. Metal
pipes are used for the main circuit and flexible hoses are used to allow for wheel
and suspension movement
• callipers
These are fitted at each wheel and operate the
brake pads. As the fluid enters the calliper at
pressure, it acts on the piston, which moves the
brake pad into contact with the disc.
• wheel cylinders
These are fitted inside the brake drum. As the
fluid enters the cylinder at pressure, the pistons
are moved out. This forces the brake shoe to
make contact with the brake drum. The diagram
shows a double acting piston type, similar to
those used on leading and trailing systems.
Single acting piston types are used for two
leading shoe systems.
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Level 2 Diploma in Light Vehicle Maintenance & Repair Principles (QCF) Unit 154 Handout 6
• servo unit
To operate the brake system, especially disc
brake, high operating forces are required. To
assist the driver most systems incorporate a
servo unit to operate the master cylinder. In
most cases these act directly on to the master
cylinder, however there are ‘remote’ types,
which are not as common, that are used to
increase fluid pressure rather than brake pedal
force.
The main type of servo unit is the ‘suspended vacuum’ type. When the brakes
are off, low pressure (‘vacuum’) is supplied to both sides of the servo piston.
When the brakes are applied, air, at atmospheric pressure, is fed to one side of
the piston. This pressure difference allows additional force to be applied and this
is transferred to the master cylinder piston. With spark ignition engines this low
pressure is obtained by a connection to the inlet manifold, but with compression
ignition engines an exhauster unit is used to create the low pressure.
The servo has to meet three main conditions:
o fail safe – if the servo assistance fails then the brakes must still operate.
This is usually achieved by allowing a direct connection to the master
cylinder
o progressive – it must allow the brakes to be gradually applied
o proportional – the application must be in a similar proportion to that
applied by the driver.
• apportioning valves
Due to the weight distribution of the vehicle, different brake forces are needed at
the front and rear. This distribution changes during braking. To distribute the
braking in the required proportion apportioning valves are used. They can be of
the following type:
o pressure limiting – these are pre-set to control the pressure front to rear
o inertia – control the pressure according to the rate of deceleration
o load sensing – controls the pressure as the weight distribution changes.
Brake circuits
Early hydraulic systems were of a single circuit; if the main system failed then the
parking (hand) brake would provide a secondary system. With these systems, a single
brake line supplied each wheel cylinder/calliper. Modern braking systems use dual
circuits and these require the use of dual or tandem master cylinders. These use a
separate circuit from each section of the master cylinder.
The dual circuits can take several forms:
• fully duplicated
These have two separate circuits for each brake system, therefore a failure on
one system would still allow all brakes to be operated
• diagonal
These have two systems but can be of different designs:
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Level 2 Diploma in Light Vehicle Maintenance & Repair Principles (QCF) Unit 154 Handout 6
1. One of the sections is connected to the NSF and OSR brake, and the
other to the OSF and NSR. If one system fails braking is still maintained
on one front brake and one rear brake. This may cause the vehicle to pull
to one side but would not be so violent as being connected to one side
only
2. One section is connected to both front brakes and the NSR, the other
section to both front brakes and OSR. Failure of one system still
maintains braking to both front wheels, but only one rear wheel. This may
cause some instability but is much less severe than the first system.
Anti-lock braking
Maximum braking is available only when the road wheel is rotating; as soon as it locks
braking will be much less efficient. Also, steering control of the vehicle will be lost as
soon as the wheel locks. Many braking systems incorporate devices to prevent the
brakes from locking.
Brake fluids
The fluid used in hydraulic brake systems has to meet certain requirements:
• must have a suitable viscosity and be able to operate down to temperatures of
approximately -400C
• must have a high boiling point (approximately 2200C)
• must have a low freezing point (approximately -650C)
• must have some lubrication properties
• must not corrode metal parts
• must not harm the seals and hoses.
The Department of Transportation sets the standards for brake fluid. Polyglycol fluids
conform to DOT 3 or DOT 4; the main difference is in the boiling point. For some
applications, a silicon based fluid is used and these will meet DOT 5 specification. Some
vehicles, such as Citroën, use a mineral based fluid; the specification for this is
completely different and cannot be used in conventional hydraulic braking systems. It is
coloured green to avoid confusion.
Brake fluids are hygroscopic, which means they absorb moisture from the atmosphere.
This moisture reduces the boiling point, which is why it is recommended to change the
brake fluid at frequent intervals. Brake fluids are incompressible; however, if any air
enters the system this will cause an increase in brake pedal travel and may result in a
total loss of brakes.
Note: Brake fluid will damage the paintwork; it must be removed immediately.
Braking terms
Brake balance
This refers to the brake forces between each wheel on the same axle. For the MOT test
the limit for brake imbalance on the steered axle is 25%.
This is calculated as follows:
Imbalance(%) = (highest brake effort – lower brake effort) X 100
highest brake effort
Example: On a roller brake test a brake effort of 50 kg is recorded for the OSF
brake and 45 kg for the NSF.
Imbalance(%) = (highest brake effort – lower brake effort) X 100
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Level 2 Diploma in Light Vehicle Maintenance & Repair Principles (QCF) Unit 154 Handout 6
highest brake effort
= (50 kg – 45 kg) X 100
50 kg
= 5 kg X 100
50 kg
= 10%
Brake efficiency
This is how well, or how efficiently, the braking system operates. If the total force applied
by the brakes was equal to the weight of the vehicle then it would have 100% stopping,
or braking, efficiency. This is calculated as follows:
Brake Efficiency (%) = total brake effort X 100
total vehicle weight
Example: A vehicle with a total weight of 2000 kg produces the following brake
force readings:
NSF: 250 kg
OSF: 270 kg
NSR: 195 kg
OSR:195 kg
Brake Efficiency (%) = total brake effort X 100
total vehicle weight
= (250+270+195+195 kg) X 100
2000 Kg
= 910 kg X 100
2000 kg
= 91000 kg
2000 kg
= 45.5%
Brake fade
When the brakes are applied the kinetic energy is converted into heat at the brake
surfaces by friction between the brake pads and disc, or the brake shoes and drums.
The friction value, known as the coefficient of friction, changes with temperature. The
hotter the braking surface becomes, the lower the coefficient of friction between the two
surfaces. Therefore, as the temperature increases past a certain point, the friction
between the surface will reduce, resulting in a loss of braking. During this stage the
brake pedal will normally remain firm, but there will be a loss of retardation.
A further effect of this is that the high temperature can cause the brake fluid to overheat
and boil, allowing air to be present. This can then lead to an increase in brake pedal
travel.
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