MAINTENANCE
Natural wear makes it necessary to perform the following maintenance
work on disc brake systems at certain intervals.
- Brake fluid check
- Brake pad replacement
- Brake disc inspection and replacement
Brake fluid check.
A brake fluid check is an essential part of any vehicle inspection. The
self-adjusting characteristic of a brake caliper means that brake pad
wear cannot be detected as a result of an increase in brake pedal
travel.
However, a drop in the brake fluid level in the fluid reservoir is a
clear indication of brake pad wear.
As soon as the level reaches the "MIN" mark, the brake system, and in
particular the brake pads, must be inspected.
Brake pad replacement.
Brake pads must be replaced when the pad friction material thickness has
worn down to 2mm or when the pads exhibit uneven wear.
Brake pads must always be replaced simultaneously on all calipers on one
axle, otherwise there is a risk that the vehicle will exhibit uneven
braking behaviour.
Use only pad sets released by the vehicle or brake manufacturer.
It is important to use only disc brake pads which are matched to the
vehicle. ATE, in conjunction with the Motor Manufacturers have performed
numerous tests, involving all driving situations, to select suitable
disc brake pads for each vehicle.
More and more brake pads are appearing on the market with special
emphasis on their wear resistance. This causes the non-technical reader
of test reports and other publications to overlook the fact that these
pads are intended only for rally and racing vehicles. Every pad reaches
its optimum coefficient of friction at a specific operating temperature.
Before this temperature is reached the coefficient of friction is
substantially different, usually lower. In normal traffic a brake is
never or only seldom subjected to stresses like those in rally or racing
vehicles. This means that the favourable operating temperature (for
achieving the optimum coefficient of friction) is never achieved. If
these "highly wear-resistant brake pads" are used in everyday operation
it must be anticipated that substantially greater foot pressure will be
required to achieve the desired deceleration, particularly during cold
weather, than is necessary in the case of pads selected by the vehicle
or brake manufacturer.
If these “highly wear resistant brake pads” are fitted on only one axle,
for example the front axle, the increased brake pressure necessary for
adequate braking of the front axle, will result in overbraking of the
rear, thereby causing the vehicle to skid and substantially increasing
the accident risk. It should also be borne in mind that when accidents
occur it is frequently maintained by way of defence that they were
caused by brake failure. This then leads to examination of the brake
system by an expert who will prove whether the brake pads installed are
matched to the vehicle type or not. Another reason for using only the
correct pads is the fact that many types of pads "grow" under the effect
of heat, which must be taken into account particularly in the case of
combined calipers with parkbrake mechanisms (automatic adjustment of
which is geared precisely to the correct brake pads.)
| Fault |
Remedy |
| Uneven pad wear |
1, 2, 10, 11, 12 |
| Brake pad exhibits slanting wear |
4, 8, 10, 11, 12 |
| Brake pads stick |
2, 11 |
| Brake pad does not retract from brake disc (brake becomes hot) |
2, 6, 10, 11, 12, 14 |
| Brake drags on one side |
1, 2, 5, 11 |
| Brake squeaks or chatters |
1, 2, 3, 6, 7, 8, 9 |
| Brake pedal travel too great |
7, 8, 9, 12, 13, 15 |
| Pistons sticking in fixed caliper |
2, 8, 11, 12 |
| No braking force |
13 |
| Pulsating effect at brake pedal, pedal flutter |
7, 8, 9 |
| Insufficient braking effect on wet roads |
1, 10 |
| Remedy N° |
| 1. |
Use specified pad type * |
| 2. |
Clean contaminated caliper interior, check protective cap ** |
| 3. |
Check 20° position of piston cut-out and adjust if necessary by
rotating piston. |
| 4. |
Replace pads with a set of specified quality pads. *** |
| 5. |
Overhaul oil-fouled brake |
| 6. |
Replace expanding spring |
| 7. |
Adjust wheel bearing clearance |
| 8. |
Check disc and caliper mounting, mutual alignment check |
| 9. |
Check disc out-of-trueness and thickness tolerance |
| 10. |
Check disc cover * |
| 11. |
Remove corrosion in fixed-caliper cylinders |
| 12. |
Check piston adjustment (seating ring) |
| 13. |
Eliminate leaks in brake system |
| 14. |
Restriction bore in master cylinder clogged, eliminate pressure in disc brake |
| 15. |
Bleed brake system |
| * |
For competition driving it may be necessary to use a different brake pad and remove the disc cover. |
| ** |
Replace brake pad and - if necessary - brake disc |
| *** |
An excessively stressed brake pad can also cause noise |
| When replacing brake discs use only Original Discs. |
| This table gives a brief overview of disc brake faults. Detailed explanations are given on the following pages. |
Excessive or uneven pad wear
Always use the specified brake pads.
The pad backing plate should slide freely within the caliper. Ensure
that the inside of the caliper is clean by wiping it with a cloth soaked
in commercially-available alcohol. Fluids containing mineral oil, e.g.
petrol, paraffin etc will damage the rubber seals and boots.
Tight pistons or pads
A
piston or pad which fits tightly in the caliper can jam although the
brake is released. The fault can be remedied by cleaning the parts in
question. If the pistons are corroded the caliper must be replaced or
reconditioned.
Unfavourable operating conditions
Pad
wear is influenced by operating conditions. It is true that snow, mud
and sand are thrown away from the disc by the centrifugal force.
However, if they are continually thrown against the disc, i.e. on dusty,
muddy or sandy roads, the disc is never totally clean and the pad is
abraded. Wear can be kept to a minimum by means of regular checks and
cleaning if it is ensured in this way that the brakes can operate
freely.
Pad does not detach from brake disc
The
pads can remain in contact with the disc after the brake is released.
This will not only cause pad wear during driving but will also lead to
an increase in the ambient temperature and thus inevitably to an
increase in operating temperature during braking. The result is a
relatively high degree of wear.
Dirt on the inside of the disc
The
inside pads usually become worn more quickly than those on the outside
as a result of the dirt thrown onto the inside of the disc. Most
vehicles have splash shields to prevent this but these shields impede
the cooling air flow across the discs. This means that there is still a
difference in wear between the inner and outer pads.
Scored or corroded discs
Heavily
scored or corroded discs can increase the friction between pad and disc
and this causes the brakes to have an aggressive effect. The disc must
be cleaned, reground or, better still, replaced.
Rear-wheel drum brakes must be adjusted
On
vehicles with disc brakes on the front wheels and drum brakes on the
rear wheels, particular attention must be paid to adjustment of the
rear-wheel brakes. If the drum brakes are not adjusted regularly they
lose their effectiveness and perform less braking work, and an increased
proportion of the deceleration must be provided by the front disc
brakes.
Excessive brake pedal travel
Excessive disc run-out, air in the brake system, unsuitable brake fluid,
distorted pad backing plates, excessive wheel bearing clearance or
damaged piston seals can cause this problem.
No braking force
Common causes are piston forced back and jammed in cylinder during
maintenance therefore no pad contact with disc, leak in system, damaged
piston seals, air in
the
hydraulic system or defective master-cylinder primary seal.
Defective adjusting mechanism
If
the case of pistons with a compensating mechanism for disc run-out, a
defective adjusting mechanism can allow the piston to retract too far
when the brake is released, resulting in increased pedal travel when the
brake is next operated. If this is the case the complete caliper must be
replaced or repaired.
Deformed shims
Some
vehicles have disc brakes with shims fitted behind or around the pads. A
deformed shim can force the piston back too far when the brake is
released and this movement must be compensated for when the brake is
next operated, resulting in turn in extended pedal travel. Bent or
damaged shims must be replaced.
Excessive disc out-of-trueness and wheel bearing clearance
Excessive disc run-out causes the disc to force the pads and pistons too
far back. This enlarges the clearance between pad and disc resulting in
turn in a longer piston stroke and thus greater pedal travel.
Spongy pedal
This
can be attributed, among other factors, to air in the hydraulic system.
The brake systems must be carefully bled.
A spongy brake pedal can also be caused by vapour bubble formation. The
brake system must therefore always be filled with the prescribed brake
fluid which must be replaced after the specified time.(2 years under
normal conditions, more often at the coast or under humid conditions).
Hard pedal
Faults which lead to this problem are common to both disc and drum brake
systems.
These are:
Defective brake booster
·
Oil or grease on the pads/linings
·
Pistons jamming in the wheel cylinders or calipers
·
Glazed or incorrect pads/linings
Excessive foot pressure
This
may be caused by the brake booster functioning poorly due to a vacuum
leak, brake fluid or grease on the brake pads, e.g.Molycote or
anti-squeal paste, jamming pistons or incorrect pads.
Pedal flutter
Pulsating effect at brake pedal
Pedal flutter during braking is usually caused by pressure waves in the
hydraulic system which can be attributed to the backwards and forwards
movement of the pistons in the calipers as the disc rotates,
or in the wheel cylinders as the brake drum
rotates. The causes
are excessive disc thickness
variation, excessive wheel bearing clearance
or in the case of drumbrakes out of round brake drums.
Brake fade
This
is highly unlikely to occur under normal
operating conditions in the case of disc brakes, which are
particularly resistant to fading, unless poor quality pads are used. The
faults listed in the section headed "Hard pedal" could also be
interpreted as fading
Heating up of brakes
Brake pad does not retract from the disc because pistons are jamming or
there is a residual pressure in the system. In the case of calipers with
park brake facility, impermissible "pad growth" - due to the use of
unsuitable pads - may be the cause.
Delayed brake response
A
defective brake booster causes a delay in
the hydraulic pressure build up
of brake system and a high pedal effort is
required before the brakes become effective. The result is that
the braking effect sets in abruptly.
Uneven braking (dragging on one side)
Most of
the factors causing uneven braking are common to both drum and disc
brakes.
They are as follows:
·
deformed discs or drums
·
worn steering linkages
·
faulty steering geometry
·
worn tyres or incorrect inflation pressure
·
loose spring bolts in suspension
·
differing types of pads on the same
axle
·
disc or pad contamination by foreign bodies is a fairly common defect
which leads to uneven operation of disc brakes. The contaminant is often
grease, oil or underbody sealant. If this occurs the disc must be
thoroughly cleaned and the pads replaced.
·
Corrosion and dirt cause jamming of the piston and pad unit, resulting
in a drop in brake performance. The result is an uneven braking effect.
If the pistons are corroded the calipers must be replaced. Embrittled,
hardened or cracked protective rubber boots
in the calliper must also be replaced. The interior of the
caliper must be cleaned using commercially available alcohol.
·
Uneven operation of shock absorbers.
·
In rubber suspension elements on the front axle worn-out or softened
rubber bearings.
·
Excessive wheel bearing clearance.
·
Brake discs replaced on one side only.
·
Defective adjusting mechanisms.
·
A defective adjusting mechanism can lead to dragging. If this is the
case it must be replaced.
Faults common to both drum and disc brakes
Further
faults which are not typical of disc brakes but which can cause the
brakes to drag are as follows:
·
loose or out-of-position wheel bearings
·
air vent in fluid reservoir screw cap blocked
·
no clearance at master cylinder push-rod
·
hydraulic lines or hoses blocked
·
brake pedal sticking
·
swollen rubber seals or cups
·
contaminated brake fluid
Loss of braking effect
Insufficient brake fluid level
Fluid leak in the system)
Fluid loss at master-cylinder secondary cups
(seals)
Fluid leak in brake units
Air
in brake system
Brakes noises (squeaking and chattering)
Chattering
Excessive disc run-out, deformed discs, excessive disc-thickness
variations, loose wheel bearings.
Squealing
Squealing can sometimes be caused by a
faulty or loose caliper mounting.
In many cases squealing during brake application can be remedied by
removing the rim of corrosion scale on the outer edge of the brake disc.
In this case the caliper must be replaced.
In the first of these two cases the caliper must be replaced, and in the second the rubber rings in the caliper.
Shims and expanding springs
Shims are used in some vehicles and are fitted between the piston and
the pad backing plate. Squealing may occur if the shims are forgotten,
warped, fitted twisted, contaminated or corroded. Missing or damaged
expanding springs can cause squealing and chattering.
Worn pads or foreign bodies in pad
Severely worn pads can also cause squealing, as can foreign bodies which
have penetrated the pad.
New pads
These can also be the cause of squealing but this will stop as soon as
the pads are run in. Brake pads which are not matched to the type of
vehicle in question may tend to squeal.
Overhang of pads on outside diameter of disc rotor.
The outer part of the pad may form an overhang if it projects beyond the
disc. In the same way an inner protuberance can form if a pad projects
too far over the friction surface of the disc into the interior of the
caliper.
Pistons twisted out of position.
Twisting of pistons in the caliper may cause squealing. Check the 20º
setting of the pistons cut-out using a piston gauge and adjust if
necessary. (On callipers with this design
feature)
Twisted pistons
Twisting of pistons in the caliper may cause squeaking. Check the 20º setting of the pistons using a piston gauge and adjust if necessary.
Disc
run-out.
·
The lateral disc run-out should be less than 0.15 mm but should not
exceed 0.20 mm even in exceptional cases.
·
If the disc is removed from the hub it is advisable to mark both the
disc and hub in order to avoid imbalance and run-out upon reassembly.
·
The disc friction surfaces should be smooth. The disc run- out can be
checked using a dial gauge, the gauge must be rigidly mounted and the
gauge pin must touch the disc at the effective radius of the caliper. ie
at the radius of the centre of the caliper piston.
·
Defective discs can be reground but should be replaced for reasons of
safety if too thin. The amount of material removed however, may under no
circumstances exceed that permitted by the vehicle manufacturer for the
vehicle type in question. This information is usually stamped or cast
onto the disc.
·
Heavily scored or distorted discs which cannot be re-faced without
exceeding the minimum permissible thickness value must always be
replaced.
·
The maximum permissible thickness tolerance (deviation of plane
parallelism) over the brake surfaces is 0.02 mm with a peak-to-valley
height of max. 5 micron.
Brake fluid
Only
Original ATE Brake Fluid or the fluid specified by the vehicle
manufacturer may be used. The brake fluids must meet the requirements of
grade DOT 3 or DOT 4 as well as those of SAE specification J 1703. Since
brake fluid is hygroscopic, use only brake fluid from a sealed
container.
Important note.
In the case of disc brakes it is advisable when driving on wet and in
particular on salted roads to operate the brake system lightly from time
to time in order to remove any residue adhering to the brake discs.
Although the centrifugal force removes the water from the brake disc, a
thin film of silicons, abraded rubber, grease and lubricating oil
adheres to the brake disc and thereby brings about substantial
reductions in the coefficient of friction, through occasional light
operation of the brake (naturally without jeopardizing other road-users)
these residues can be removed from the edges of the brake pads and
burned off, thereby guaranteeing immediate brake system response in the
event of an emergency.
Before the vehicle is parked the brake system should be operated several
times so that all parts of the system can dry off and any dampness
removed.
If the vehicle has been parked following a journey in rain, snow or
slush, the brake should be operated several times with the necessary
caution when driving is resumed in order to restore effective braking
action.
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