LV32 braking systems (2)

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Student Workbook

LV32 Braking Systems (2)

kap all phase 2 & 3 6/11/03 11:37 am Page 39

Student Workbook for Technical Certificates in

Light Vehicle Maintenance and Repair

MODULE LV32 BRAKING SYSTEMS (2)

Contents

Page ……… Page

When diagnosing brake problems it is important to consider other systems on the vehicle that may when faulty, give similar symptoms to those of the brake system

With this in mind, consideration should be given to the following: -

• The first inspection should be of the wheels Look for any damage to the rim that could cause buckling Check the tyre inflation pressures and wear patterns Finally check the adjustment of the wheel bearings

• Move onto the inspection of the suspension system, checking all

components for signs of wear or damage

• Wheel alignment problems can often result in the incorrect diagnosis of the brake system For example if the camber or caster had an out of tolerance problem, then this could cause a vehicle to pull to one side in a similar

manner to a brake problem

• Once the other vehicle systems can be ruled out of a diagnosis process

then we can start to look at the brake system itself

• hard pedal but inefficient braking

• squealing or chattering when brakes applied

Low or spongy pedal

When the brakes are applied the pedal travel is excessive It nearly touches the floor and in some cases actually does The pedal may also feel ‘spongy’ and the brake’s efficiency is significantly reduced

Possible causes

Brake pedal height adjustment

The brake servo push rod if incorrectly adjusted can be a cause of a low brake pedal height It can usually be adjusted by removal of the brake light switch and adjusting the push rod length via a locknut Then refit the brake light

switch and ensure correct operation of the brake lights

Brake pedal freeplay too large

Brake pedal freeplay can also be a common cause of a low brake pedal

height Stop the engine and pump the pedal until no vacuum is left in the

servo Push in the pedal by hand until a resistance is felt Measure the

distance, (usually about 2-6m, but this can vary from car to car) If the

freeplay is non-existent or excessive, check the adjustment on the brake light

switch.

Lining to drum clearance is excessive

Remove the brake drum Using a vernier caliper, measure the internal

diameter of the brake drum and then measure the external diameter of the

brake shoes Take the shoe diameter away from the drum diameter and what

is left will be the clearance between the brake shoes and the brake drum The usual value would be 0.6 mm; anything larger and the pistons inside the wheel cylinders would have to travel further to push the linings against the drum

This would result in the brake pedal having to travel further to supply the

required amount of pressurised brake fluid The easiest way to rectify this

problem would be to adjust the brake shoe to brake drum clearance There are various methods but most use some sort of ratchet or adjusting rod

Fluid leaking from the hydraulic circuit

Modern vehicles use a tandem master cylinder, which means the hydraulic

circuit is split into two circuits for safety reasons If one circuit fails, (i.e leaks) then the other circuit can still operate two of the four brakes This does

however result in the brake pedals travel increasing significantly

Causes of brake fluid leaks could be:

• damaged/worn wheel cylinder seals

• damaged flexible hoses

• loose pipe unions

• faulty master cylinder

• faulty load sensing proportioning valve

Air in the hydraulic system

When air is trapped in the system it will affect the brake pedal ‘feel’ The more air the more spongy the pedal When the brake pedal moves hydraulic

pressure increases within the brake system

In a normally operating system the brake fluid cannot compress and therefore can only move the brake callipers/wheel cylinders and therefore the brake pads

On a system that contains air, the air compresses so that not all the movement

is transferred to the brake calipers and wheel cylinders The driver has no other choice than to continue pressing the brake pedal until the brakes are applied sufficiently This results in excessive brake travel

Air in the brakes MUST be removed and this is achieved by bleeding the

brakes Usually this job is carried out by two technicians, however there are various devices available to make it possible for a single person to carry out the task The basic principle was to have one technician sitting in the vehicle whilst the other carried out the bleeding operation The technician bleeding the

brakes would connect a tube to the furthest bleed nipple from the master

cylinder Once it had been loosened off, the person in the car would pump the brake pedal and the displaced fluid would travel down the tube to be collected

in a suitable container During this operation it is important that the master

cylinder reservoir is not allowed to become empty Once the ejected fluid runs clean, the nipple is tightened up and the technician repeats the procedure on the other three wheels

Brake disc runout and/or thickness variation

If the brake disc has excessive runout (warped) as it turns its overall thickness will increase and therefore the brake pads will be pushed back into the caliper When the driver pushes the brake pedal the pads have further to travel before they come into contact with the brake disc Therefore the brake pedal travel is excessive If the brake disc has a thickness variation, then the same problem will occur

The procedure to follow if the above is suspected would be:

• check wheel bearings for free play

• adjust/replace wheel bearings as necessary

• check brake disc for runout and thickness variation

After removing the front wheels, refit the wheel nuts to ensure that the brake disc is clamped correctly Use a magnetically mounted DTI, (Dial Test

Indicator) Set pointer to run on the brake disc surface approximately 10 mm in from the edge Turn the disc until the highest or lowest point is found and set the DTI to zero Turn the brake disc by hand slowly and observe the runout

On an average family car the runout should not exceed 0.05 mm However refer to the manufacturers’ literature where available

Once this test has been carried out, regardless of the result, a thickness

variation test should also be carried out This will give you a clear picture of what the actual fault is For example the brake disc could and probably will have an element of both problems

To carry out the thickness variation test it will probably be easier to remove the brake caliper Once this is done mark a line, with a marker pen 10 mm in from the edge of the disc Now draw four lines across the centre of the disc This will give you eight evenly spaced zones With a micrometer measure the brake disc thickness at the eight different positions Compare the readings to give you an idea of disc thickness variations

Resurfacing brake discs

Assuming that the brake discs thickness is not too near, or below the

manufacturers’ minimum thickness (usually stamped on the disc), the brake disc can be re-machined (turned in a lathe) There are two methods of doing this One is the ‘on car’ method, whilst the other is the ‘off car’ method On the car means fitting an on car lathe to the brake caliper mounting points that will

machine the disc almost perfectly, taking into consideration any slight runout in the wheel hub Off car usually means removing the discs and sending them to a local machine shop where they would be turned on a lathe This method is not as accurate, as any hub distortion is not taken into consideration

Vapour lock

When the foot brake is constantly applied such as on a mountainous road, the brake linings will become very hot This is transmitted through to the brake

fluid, which also gets very hot

Vapour lock refers to the condition where the brake fluid in the brake lines

reaches boiling point and air bubbles are produced As previously stated,

when air is present in brake hydraulic circuits, it is compressed and the brake pedal travel increases This results in the brake efficiency being reduced

The amount of moisture that brake fluid contains has a direct affect on its

boiling point, as brake fluid has a tendency to absorb moisture from the

atmosphere (hygroscopic) The older the fluid is the greater the possibility for the vehicle to suffer from vapour lock The easy remedy is to replace the

brake fluid on a regular basis, according to manufacturers’ instructions,

(generally every two years, 20,000 miles) with a good quality brand

Progress check 1

Answer the following questions:

1 Name three possible causes for a ‘spongy’ brake pedal:

2 How would you rectify ‘excessive brake freeplay’?

3 Name three other components on the vehicle that when faulty, could cause brake like symptoms:

4 What can go wrong with a brake master cylinder?

5 When repairing a vehicle with excessive brake disc runout which of the

following repairs would be the BEST option and why?

a) Replace brake discs

b) Re-machine the brake discs on an off car lathe

c) Re-machine the brake discs on an on car lathe

6 What word do we use to describe water absorption of brake fluid?

a) Hydroscopic

b) Hygroscopic

c) Hypermarket

Brakes drag

Brakes drag can be described as follows:

Whilst driving, the vehicle seems to be resisting movement If the vehicle is allowed to coast, it slows as if the brakes are being applied Stopping on a hill the vehicle may remain stationary without the hand brake/foot brake being

applied

Possible causes

No pedal free play

As previously discussed, brake pedal freeplay is measured at the brake pedal itself If there is no freeplay then the compensating ports, (fluid drain back to the reservoir) in the master cylinder are not open and therefore the brakes

remain slightly on The brakes get hot because of the constant binding of the linings and so the fluid gets hot which expands and increases the braking

force Eventually the vehicle comes to a stop because the brakes are fully on The way to check/adjust this is as stated earlier

Parking brake dragging

The handbrake may drag/bind as a result of:

• brake shoes being over adjusted

• hand brake cable being over adjusted

• hand brake cable being seized

• linkages within the brake drum being seized

• wheel cylinder sticking

The best approach would be to remove the brake drums, (re-adjusting the

handbrake cable may be necessary) and check that the linkages within the

drum are free to operate If you ask someone to operate the hand brake lever

on and off repeatedly, you can observe the handbrake operation on BOTH

sides of the vehicle Make sure that when the hand brake is released, the

mechanism returns fully

Wheel cylinder or brake caliper sticking

Check the brake calipers for sticking; it should be possible to move the caliper from left to right as shown by hand If not then the caliper slides are likely to

be seized To check the wheel cylinder, remove the brake drum and use two screwdrivers to move the brake shoes/wheel cylinder pistons left to right

ensuring they are not sticking

Brake pads sticking

Check pad mountings are not sticking

If the brake pads stick to the caliper housing, then the brakes will drag It is important to check that the pads are free to slide If they are not then the

pads should be taken out of the mounting and any rust removed, before

applying high melting point grease to the contact surfaces and re-assembling

Residual pressure to high

During normal operation of the brakes, when the brake pedal is released the master cylinder pistons move all the way back, due to spring pressure and

uncover the compensating port This port allows any residual pressure/brake fluid in the brake system to return to the reservoir

In between the brake servo (booster) push rod and the master cylinder piston there should be a small air gap, (A) This gap is to ensure the master cylinder piston moves back far enough to uncover the compensating port If the

compensating port is not allowed to open then the brake system will hold a

small amount of fluid pressure, which is usually enough to apply the brakes slightly Applying the brakes constantly will create heat and will warm up the brake fluid, which will expand Unfortunately due to the blocked

compensation port this expanding fluid has nowhere to go This results in the brake calipers/wheel cylinders having to move and therefore applying the

brakes further, finally bringing the vehicle to a halt

Servo (booster) adjustment

Different manufacturers use different special tools to carry out this job

However the basic principles are the same Looking at the special tool above, you can see that the ‘SST rod’ can slide up and down through the main body

of the SST, (Special Service Tool) First of all put the tool onto the master

cylinder, (diagram above left) and push the SST rod until it just touches the

piston, (ensure the piston does not move) Turn the tool over and place it on the brake servo as shown above Adjust the pushrod until it just touches the end of the SST (i.e no gap) The adjustment is now done As the SST rod is slightly longer than the rest of the SST, this will result in a small air gap when the servo/booster is coupled back up to the master cylinder

Distorted brake drums or shoes

Brake drums or brake shoes with ‘high’ spots on them can cause the brakes

to drag Using a vernier caliper measure the brake drum and shoes at various places to ensure they are not out of round

Brakes disc or drum distorted

Distorted brake drums and discs can cause the vehicle to pull to one side or the other The rectification would be to measure and resurface the discs or drums as described earlier

Sticking brake caliper

Sticking brake calipers can also cause the vehicle to pull to one side Check

as previously described

Brake pads sticking

Check pad mountings are not sticking

Check the brake pads are not seized in the brake caliper Remove any rust and apply high melting point grease to the contact surfaces, (pads – caliper carrier)

Resistance between the brake shoes and backplate

Ensure that the brake shoes are free to slide on backplate Apply high melting point grease at contact points

Faulty return spring

Check the return spring is not broken or weak

Progress check 2

Answer the following questions:

1 Explain in your own words why the servo push rod adjustment could, if

carried out incorrectly, cause brake drag?

2 What method could you employ to check that the wheel cylinders and

brake calipers are not seized?

3 Name two possible brake problems that could cause a vehicle to pull to the right:

Brakes grab

When the brake pedal is pressed slightly, the vehicle suddenly brakes heavily

or shudders, as the brakes appear to engage and disengage, repeatedly

Possible causes

Check linings for contamination

If the brake linings are contaminated by oil, grease or even water, the brakes will tend to grab If under inspection the linings are found to have

contamination of any kind then they should be replaced

Disc brake or drum distorted

If the brake drums are out of round or the brake discs are warped, then it is

quite likely that brake grab may occur Rectification is as mentioned earlier

Wheel cylinder loose

If the wheel cylinder becomes loose the brake shoes will not be applied

smoothly, which results in the brakes grabbing

Brake servo faulty

If the brake servo develops a fault, it can under some circumstances apply

uneven pressure to the master cylinder This in turn will cause the brakes to grab

Brake servo test - (operation test)

To test the operation of the servo is quite simple There are basically two

tests, the first one being an operation test This as the name describes, tests the operation of the servo It is carried out as follows:

• turn the engine off and pump the brake pedal a number of times until the servo has lost its vacuum

• apply a constant pressure to the brake pedal and start the engine up

• the brake pedal should drop down as the servo assists your foot

Brake servo test - (air leaks)

The second test is an air tightness test (air leak test):

• run the engine for 1-2 mins and then stop it

• when the brake pedal is pressed repeatedly check that with each operation the pedal moves closer to the floor

• start the engine again and run it for at least two minutes

• with the engine still running press and hold the brake pedal

• stop the engine whilst still holding the brake pedal and hold it with a

constant pressure for 30 seconds

• after 30 seconds ensure the pedal position has not changed

Proportioning valve faulty

The proportioning valve, as discussed on previous brakes modules is a device that is designed to control the amount of braking the rear wheels carry out As the vehicle’s weight is thrown forward during braking it is important to reduce the amount of rear braking, as there would be a tendency for the rear wheels

to lock up If this valve was to fail then it is possible for it to cause excessive braking to be applied to the rear brakes

Hard pedal but inefficient braking

Whilst operating the brakes the pedal feels good but the vehicle seems

reluctant to slow down or stop This results in the driver having to apply more force to the brake pedal to get the vehicle to slow down and stop

Possible causes

Water adhering to the brake drums/discs

When a vehicle’s brakes get flooded, for example, when it is driven through a ford or stream the friction surfaces become wet and the coefficient of friction drops, which means the surface becomes more slippery This results in a

hard brake pedal but ineffective braking Brake drums are more prone to this problem as they tend to trap the water, whereas brake discs disperse the

water far more quickly The remedy is simple; apply brakes lightly when they are wet to dry them out before you actually need to brake properly

Contamination of brake linings

If oil or grease contaminates the brake linings, then the brakes will tend to

lose their effectiveness If under inspection the linings are found to have

contamination of any kind then they should be replaced

Proportioning valve blocked or sticking

If the proportioning valve were to get blocked then the driver would lose the use of their rear brakes This would, without doubt be felt by the driver as a hard brake pedal but inefficient brakes

Faulty brake servo

Without the aid of the brake servo, the brake pedal would feel very hard but would be almost total ineffective

Test as previously discussed

Brake fade

Brake fade is usually caused by excessive use of the brakes For example:

• towing caravans, boats etc

• long downhill descents, such as mountainous roads etc

• motor sport

The problem is caused by the fact that as brake linings heat up, the coefficient

of friction between the linings and the discs drop, which basically mean the

linings lose their ‘grip’ on the disc

The remedy is to use engine braking where possible or install bigger brakes or

a more suitable brake pad compound

Squealing or chattering when brakes applied

Brake friction materials inherently generate noise and heat in order to

dissipate energy Therefore it has to be expected that an occasional squeak will be generated It is up to the technician to determine the level of

acceptability between similar cars However some external influences can

increase the noise to an unacceptable level