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ANALYSIS OF BRAKE SPONGY DEFECT IN
COMMERCIAL VEHICLE AND
DEVELOPING THE COUNTER MEASURES.
CASE STUDY
Abstract
Analyses identified that there were as many as 11 important causes for brake
spongy defect in vehicles, and details of these causes are;
- tightening defect, more clearance between brake pad and rotor, more
clearance between brake shoe and brake drum, brake and clutch pedals in low
position before filling the brake fluid, improper clamping of gun seal packing’s
to the reservoir, damage in gun seal packing’s of brake fluid filling gun,
presence of moisture or dust in the brake tube, improper clamping of pressure
seal clamp between gun to hose joint, blockage of vacuum filter, presence of
moisture in brake fluid and bad brake hose material.
Paper discusses the sources of these causes and identification of counter
measures.
Keywords:
Brake system, Brake fluid, Brake spongy.
1. Brake Spongy
Brake spongy is the soft feeling of brake pedal, that means brake pedal travel
before any action takes place in brake system. It should be between 1 to 3 mm, if
brake pedal free play is greater than 3 mm it causes brake spongy. Brake spongy
is related to forces in braking system, brake system components, brake fluid
filling equipment, which are briefly discussed here under.
1.1 Forces in Braking System
The braking force applied and pressure developed inside brake system is
determined by Pascal's law which states that "pressure exerted anywhere in a
confined incompressible fluid is transmitted equally in all directions throughout
the fluid such that the pressure ratio remains the same". From basic physics, the
kinetic energy of a body in motion is defined as: Kinetic Energy = ½mv, m is
mass and v is velocity of the vehicle.
Brake pedal exists to multiply the force exerted by the driver’s foot, the force
increase will be equal to the driver’s applied force multiplied by the lever ratio

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of the brake pedal assembly.
Master cylinder translate the force from the brake pedal assembly into hydraulic
fluid pressure.

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1.2 Brake System Components
It consist of Brake booster, Master cylinder, Disc Brakes, Drum Brakes. The
working of these components of brake system are explained hereunder.
Brake Booster: The typical brake booster shown in Fig. 1; the basic principle of
the brake booster is pressure differential, when vacuum is applied to both sides
of the diaphragm piston it is pushed to the right by the spring and remains there.
When the brake pedal is pressed it causes the plunger to move inside brake
booster and atmospheric air is allowed into variable pressure chamber. Due to
the difference in pressure the piston starts to compress the spring and moves to
the left. This causes the piston rod to move the piston of the master cylinder and
generating hydraulic pressure.
Brake Booster

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Master Cylinder: The typical master cylinder is shown in Fig. 2; when the brake
pedal is de-pressed, the primary piston moves to the left. As the primary piston
is pushed farther it builds hydraulic pressure inside the master cylinder which is
transmitted to the wheel cylinders of the rear brake. The same hydraulic pressure
is applied to the secondary piston, and hydraulic pressure in the primary
chamber moves the secondary piston to the left, after the compensating port of
the secondary chamber is closed, hydraulic pressure builds and is transmitted to
the brake caliper of front brake.
Master Cylinder with Reservoir

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Drum Brakes: It consist of wheel cylinder, brake shoes shown in Fig. 3; When
the hydraulic pressure builds inside the master cylinder, it is transmitted to the
wheel cylinder and in turn it pushes piston on each end of the wheel cylinder
outward, this in turn pushes the brake shoes against the brake drum, Braking
force is generated by friction between the brake shoes and the lining of brake
drum.
Brake drum

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.
Disc Brakes: The disc brake assembly consists of rotor that rotates with the
wheel, Caliper assembly, brake pad that is mounted to the caliper assembly
shown in Fig. 4. When the hydraulic pressure builds inside the master cylinder,
it is transmitted to the brake caliper and in turn it pushes the brake pad to contact
the disc.
Disc Brake
1.3 Brake Fluid Filling Equipment
The brake fluid filling equipment is consist of De-aerated tank, Cushioning tank,
De-gasified, Drain salvage tank, Vacuum tank Brake fluid equipment has a
optical fiber sensor which is placed inside the brake fluid barrel and it sense the
level of brake fluid then indicate the pump to supply brake fluid to the de-
aerated tank. De-aerated tank has a provision for de-gasified where the sprayed
brake fluid is collected and condensed then it is passed to the de-aerated tank.
Brake fluid from de-aerated tank is passed to the cushioning tank where it is
stored before sending it to the reservoir of vehicle. The following are the
functions of brake fluid filling equipment:
Pre-vacuuming: Vacuum pump creates 9.9x10
2
pa vacuum pressure within 10
seconds when the gun is clamped to the reservoir of the vehicle.
Vacuuming: It is the continued process of the Pre-vacuuming where in vacuum
pressure from 9.9x10
2
pa to 2.1x10
2
pa is created within 70 seconds.
Filling: Filling is done after the vacuum pressure is set to required level inside
the reservoir of the vehicle, then the pressure regulator valve develops the 2x10
5
pa pressure inside the cushioning tank that forces the brake fluid inside the
cushioning tank to fill brake fluid inside the reservoir and filling takes place in
28 seconds.

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Leveling: In this process the vacuum of 2.4x10
2
pa is created inside the drain
salvage tank by the vacuum pump for 10 seconds it removes the excess brake
fluid from the reservoir and send it to drain salvage tank.
Blow: After leveling process, the gun has to be unclamped from reservoir. It is
not easy to unclamp the gun as it causes damage to reservoir due to vacuum
pressure, for this purpose vacuum pressure has to be removed. The pressure
regulator valve supply the pressure of 2 bar within 10 seconds to overcome
vacuum pressure.
1.4 Types of Error in Brake Fluid Filling Equipment
Types of Error in brake fluid filling equipment are given in the Table 1. Out of
these errors only the Vacuum and Pre-vacuum error causes low or no brake fluid
filling condition. Only these errors are considered for further investigation and
analysis.
Table 1 Types of Error in Brake Fluid Filling Equipment
Types of Contribution
of error in
Machine
Condition
defects
percentage
Vacuum
error 47.06%
Low brake
fluid filled
Pre
Vacuum 19.85%
No brake
fluid filled
error
Other 14.71%
Not
concern
about
brake fluid
ABS fault
&
Not
concern
aboutABS
actuator 10.29%
brake fluid
error
Filling
fault 8.09%
No brake
fluid filled
or
Low brake
fluid filled

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2. Identification of Causes and Developing Counter
Measures for Brake Spongy
Detail investigative study was carried out to identify the causes of brake spongy,
these causes were classified under four categories such as Man, Machine,
Materials and Methods. Fig.6 gives details of all possible causes identified under
different categories responsible for brake spongy defect.
Brake Fluid Filling Equipment

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For each cause the 5 why analysis is used to explore the cause and effect
relationships underlying a particular problem and developing the counter
measures. The primary goal of 5 why analysis technique is to determine the root
cause of a brake spongy defect by asking question why for 5 times and find the
answers for these questions which will be helpful in developing counter
measures. Due to space limitations 5 why analysis and identification of
countermeasures for the cuases due to ‘Man’ are explained in the paper.

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Cause and Effect Diagram for Brake Spongy
2.1 Five Why Analysis – Brake Spongy Defect due to Man
It explains how the brake spongy defect occur from worker during assembling
the brake system components using 5 Why Analysis, Accordingly 5 why
possible questions were identified as given below.
(i) Why? – there is improper tightening of Brake tube
The tightening defects in brake tube causes due to improper apply of torque,
cross thread and poor seating of double flare, which are described hereunder.
Torque: the torque should be in correct range while tightening brake tube if the
worker applied torque is less then there will be gap between the double flare and
tubing seat shown in Fig. 7, due to this there will be leakage of vacuum pressure
which causes pre-vacuum error. Torque should be between 9.5 Nm to 22.5 Nm
and target value is 16 Nm.
Cross thread: while tightening brake tube if the worker tilts the flare nut this
causes cross thread, due to this there will be leakage of vacuum pressure and
causes pre-vacuum error.
Poor seating: while tightening brake tube if the worker does not place the
double flare of brake tube to seat properly on tubing seat, there will be gap
between the double flare and tubing seat due to this there will be leakage of
vacuum pressure which causes pre-vacuum error.
After detail analysis and observation of workers doing the assembly of brake
tube it is identified that “tightening defect” is one of the sources or causes of
brake spongy defect.
(ii) Why? – More clearance between Brake Pad and Ro tor occurs
During assembling front disc brake the worker adjust the clearance between
brake pad and rotor through the caliper. Due to improper alignment of caliper by
the worker it causes more clearance between the brake pad and rotor. A sample
data shown in Table 2 shows that out of 10 vehicles randomly inspected for
clearance between brake pad and rotor it is found that in vehicles the clearance
is more in many vehicles. This results in more distance for the brake pad to
touch the rotor. Because of this more brake force is required for brake pad to
touch the rotor. If the brake pad does not touch the rotor it causes brake pedal
free play more than 3 mm due to this brake spongy occurs.
While observing workers assembling the front disc brake it is identified that

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“more clearance between Brake Pad and Rotor” is one of the sources or causes
of brake spongy defect.
(iii) Why? – more clearance between Brake Shoe and Brake Drum occurs
During assembling rear brake drum the worker adjust the clearance between
brake shoe and brake drum. Due to improper alignment of brake shoe and brake
drum by the worker it causes more clearance between the brake shoe and brake
drum. This results in more distance for the brake shoe to touch the brake drum
so more brake force is required for brake shoe to touch the brake drum. If the
brake shoe does not touch the brake drum it causes brake pedal free play is more
than 3mm due to this brake spongy occurs.
Analysis and observation of workers assembling the rear brake drum it is
identified that “more clearance between Brake Shoe and Brake Drum” is also
one of the source or cause of brake spongy defect.
(iv) Why? – Brake pedal or Clutch pedal is in low position before
filling Brake fluid
Due to negligence of worker, the brake pedal or clutch pedal is kept in low
position. In turn this result in low brake fluid filled in brake system
components.
(a) Brake pedal is in low position before filling Brake fluid
When the brake pedal is in upper position, the plunger remains in same position
inside the brake booster this causes push rod of brake booster to remain in same
position. In turn push rod of brake booster does not move the primary piston, due
to this both primary inlet port and primary compensating port remains in open
position. It causes the secondary piston remains in same position, due to this
both secondary inlet port and secondary compensating remains in open position.
During vacuuming process, it creates vacuum pressure in all brake components
since both primary and secondary compensating port is in open position and in
turn brake
Due to negligence of worker, the brake pedal is kept in low position. It causes
the plunger to move inside brake booster. In turn it pushes the push rod of brake
booster and push rod of brake booster moves primary piston inside the master
cylinder, due to this primary inlet port is in open position and primary
compensating port is in closed position. In turn primary piston moves secondary
piston causing only secondary inlet port to open and secondary compensating
port to close. During vacuuming process, the vacuum pressure is not created
inside the rear wheel cylinder and front disc brake since the both primary and
secondary compensating port is in closed position and due to this brake fluid is
not filled to rear wheel cylinder and front disc brake.

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fluid is filled to all brake components.

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(b) Clutch pedal is in low position before filling Brake fluid.
The brake fluid reservoir is connected to clutch master cylinder (CMC) through
a hose. When the clutch pedal is in upper position, the push rod remains in same
position. In turn push rod does not move the piston inside the CMC, due to this
both inlet port andcompensating port remains in open position. During
vacuuming process, it creates vacuum pressure in all components since
compensating port is in open position and in turn brake fluid is filled to all
components.
Due to negligence of worker, the clutch pedal is kept in low position. It causes
the push rod to move inside CMC. In turn push rod moves piston inside the
CMC, due to this inlet port is in open position and compensating port is in
closed position. During vacuuming process, the vacuum pressure is not created
inside the clutch tube and release cylinder since the compensating port is in
closed position and in turn brake fluid is not filled to clutch tube and release
cylinder.
With the above discussion it is found that “Brake pedal is in low position before
filling Brake fluid” and “Clutch pedal is in low position before filling Brake
fluid” are also likely sources of brake spongy defect.
(v) Why? – Gun seal packing’s of brake fluid filling gun is not clamped properly
Gun seal packing A and gun seal packing B shown in Fig. 8 is used for sealing
brake fluid reservoir so that vacuum pressure does not leak while creating
vacuum, if worker handling brake fluid filling gun does not clamp it properly to
the brake fluid reservoir then leakage of vacuum pressure occurs, in turn this
leads to pre-vacuum error.
It is found that “Improperly clamped gun seal packing’s of brake fluid filling
gun” causes pre – vacuum error that leads to brake spongy defect.
2.2 Identification of Counter Measures
As mentioned earlier details of 5 – why analysis and identification of counter
measures related causes of ‘Man’ is explained in the paper. Accordingly counter
measures identified for causes due to ‘Man’ are explained here below.
(i) Tightening defect of Brake Tube
Tightening defect causes the leakages of vacuum pressure, due to this pre–
vacuum error occurs. This hap pens when the vacuum pressure of 9.9x10
2
pa
created by vacuum pump does not remain same for 10 seconds. Pressure sensor
measures drop of vacuum pressure, the drop of vacuum pressure indicates there
is a leakage of vacuum pressure. As discussed in 5 - why analysis presented in
section 2.1 tightening defect of Brake tube occurs due to improper apply of

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torque, cross thread and poor seating of double flare assembly. As a counter
measure to overcome, this problem it is recommended that after the assembly of
Brake Tube a quality inspector is employed for double check whether torque of
different component mentioned in the Table 3 is achieved by the worker, Torque
should be between 9.5 Nm to 22.5 Nm and target is 16 Nm so the double check
is done by using Auto-torque tool and is set to the lower limit 9.5 Nm, if the
component moved while tightening it indicates that it is not tightened properly.
(ii) More clearance between Brake Pad and Rotor.
As per the company standard, brake pedal free play should not exceed 3 mm. In
order to limit the Brake pedal free play at 3mm it is required to find out optimum
clearance between brake pad and rotor. Following procedure is used to calculate
the optimum clearance.
Calculation of Clearance required between Brake pad and Rotor for Brake pedal
free play upto 3 mm.
When the force applied to the brake pedal pad by the driver the output force of
brake pedal assembly increases and will be equal to the driver applied force
multiplied by the lever ratio of brake pedal assembly.
(iii) Brake pedal and Clutch pedal is in low position before filling
Brake fluid
While assembling the compounds inside the vehicle, the worker carries the tray
which consists of the components that is to be assembled. Due to the negligence,
if the worker places the components tray on the brake pedal or clutch pedal it
pushes brake pedal or clutch pedal to low position before filling brake fluid.
This leads to less quantity of brake fluid filled to reservoir. To avoid such
mistakes of workers at Final 1 line of assembly line as to be educated not to
place the tray on the brake pedal and also make sure it is in correct position.
(v) Improperly clamped gun seal packing’s of brake fluid filling gun
If the worker improperly clamps gun seal packing’s of brake fluid filling gun it
results in leakage of vacuum pressure which causes pre- vacuum error. Hence
the workers are to be trained to place the gun seal packing’s appropriately clamp
to the reservoir.

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2.3 Causes due to Machine
Cause: Damage in gun seal packing’s of brake fluid filling gun
Counter Measure: If there is damage in gun seal packing’s of brake fluid
filling gun, it leads to leakage of vacuum pressure and this also causes pre-
vacuum error. In this case if pre - vacuum error occurs, it is difficult to identify
for what reason it happened. To identify this problem the mock model of
reservoir is made by closing the three ports excluding to port and top port is used
for placing brake fluid filling gun to check whether vacuum pressure drops or
not after developing vacuum pressure inside the mock reservoir. If the pressure
sensor detects the drop in vacuum pressure this indicates there is a problem in
gun seal packing’s and it as to be replaced.
Cause: Improper clamping of pressure seal clamp between Gun to hose joint
Counter Measure: Pressure seal clamp is used for joining the gun and hose
shown in Fig. 10.Due to improper clamping of pressure seal clamp between gun
to hose joint, which results in leakage of vacuum pressure between gun to hose
joint, due to this pre- vacuum error.
Pressure seal clamp should be clamped properly between gun to hose joint and
also check once in a day whether the leakage of pressure vacuum occurs.
Cause: Blockage of Vacuum filter
Counter measure: Vacuum filter in the vacuum hoses is mounted in-line on the
vacuum hose. Accumulation of dust particles inside the vacuum filter causes
blockage of vacuum filter. During vacuuming, if the Vacuum filter is blocked
this causes vacuum pressure to block so the vacuuming is not done to brake
system components which results in vacuum error.Vacuum filter has to be
checked daily and if there is accumulation of dust, clean it. Vacuum filter should
be cleaned once in a week by using distilled water and dried before placing it
back.
2.4 Causes due to Method

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Cause: Presence of moisture or dust in the Brake tube
Counter Measure: During vacuuming, if moisture or dust is present in the
brake tube it causes the blockage of brake tube, due to this blockage vacuuming
is not achieved in farther component and which results in vacuum error. To
overcome this problem after removing the cover of brake tube, air gun used to
blow2 bar pressure inside the brake tube to remove dust or moisture present in it
and end cap is place on the both end of brake tube.
Cause: Presence of moisture in Brake fluid
Counter measure:If the moisture is present inside the brake fluid, it decreases
the boiling point of fluid and results in the vaporization at lower temperature
which causes brake spongy. Vapor lock point equipment is used for detecting the
boiling point of brake fluid and from boiling point we can detect the moisture
content. Boiling point of brake fluid should be determined whenever the brake
fluid barrel is changed.
2.5 Causes due to Material
Cause: Bad Brake hose material.
Counter measure: During vacuuming, if brake hose material has defect like
crack or hole it causes leakage of vacuum pressure, this results in pre-vacuum
error. Due to defective brake hose material it also causes inflate of brake hose at
higher pressure. When brake pedal applied this causes expansion of brake hose
and results in accumulation of brake fluid inside the brake hose. Due to
accumulation of brake fluid it damages the brake hose resulting crack on brake
hose, this leads to leakage of brake fluid.

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3. Conclusion
A comprehensive root cause analysis using 5 - Why methodology to find the
causes and reasons for brake spongy defect resulted in identifying suitable
counter measures for this problem. Out of 20 suspected causes for brake spongy
defect 11 causes are identified as potential sources of this problem. The list of
these 11 causes and corresponding counter measures is given below.
(i) Tightening defect: Torque range for tightening brake tube is standardized
and worker is employed to check whether the torque for tightening different
component is achieved.
(ii) More clearance between Brake pad and Rotor: This clearance is to be ≤
0.3066 mm.
(iii)More clearance between Brake shoe and Brake drum: This should be ≤
1.3055 mm
(iv)Brake and Clutch pedals in low position: The team member is educated not
to press the brake pedal and clutch pedal by putting the components tray in
pedals and ensure that the pedals are in correct position before filling the
brake fluid.
(v) Improper clamping of gun seal packing’s: Workers are to be trained to place
the gun seal packing’s properly and appropriately clamp to the reservoir.
(vi)Damage in gun seal packing’s of brake fluid filling gun: Mock model of
reservoir is developed to check the pre-vacuum error.
(vii) Improper clamping of pressure seal clamp between gun to hose joint:
Pressure seal clamp should be clamped properly between gun to hose joint,
and also pressure seal clamp should be checked once in a day whether the
leakage of pressure vacuum occurs.
(viii) Blockage of Vacuum filter: Vacuum filter has to be checked daily and if
there is accumulation of dust, clean it. Vacuum filter should be cleaned once
in a week by using distilled water and dried before placing it back.
(ix) Presence of moisture or dust in the Brake tube: Air gun is used to
blow 2 bar pressure inside the brake tube to remove dust or moisture and
end cap is place on the both end of brake tube.
(x) Presence of moisture in Brake fluid: Vapor lock point equipment is
used for detecting the boiling point of brake fluid.
Bad Brake hose material: Brake hose should not inflate at pressure less than
6.045N/m^2.
Reference
[1] JAMES WALKER, 2005, The Physics of Braking Systems, copy right of
Stop Tech LLC, United States, 1-8.

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