DIAGNOSIS AND TESTING
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The brake system operates by transferring effort applied to the brake pedal
by the driver to the brakes at each wheel.
The braking effort is distributed to each wheel, using a hydraulic system. The system is assisted using a vacuum brake booster that reduces pedal effort and increases hydraulic pressure. The parking brake operates on the rear wheels and is applied using a hand operated control.
Inspection and Verification
NOTE: Before carrying out any diagnosis, make sure that the brake
system warning indicator is functional.
Visual Inspection Chart
Road Test
Carry out a road test to compare actual vehicle braking performance with the
performance standards expected by the driver. The ability of the test driver to
make valid comparisons and detect performance deficiencies will depend on
experience.The driver should have a thorough knowledge of the brake system operation and accepted general performance guidelines to make good comparisons and detect performance concerns. An experienced technician will always establish a route that will be used for all brake diagnosis road tests. The road selected will be reasonably smooth and level. Gravel or bumpy roads are not suitable because the surface does not allow the tires to grip the road equally. Crowned roads should be avoided because of the large amount of weight shifted to the low set of wheels on this type of road. Once the route is established and consistently used, the road surface variable can be eliminated from the test results. Before a road test, get a complete description of the customer concerns or suspected condition. From the description, the technician will be able to match possible causes with symptoms. Certain components will be identified as possible sources while others will be eliminated by the evidence. More importantly, the customer description can reveal unsafe conditions which should be checked or corrected before a road test. The description will also help form the basic approach to the road test by narrowing the concern to specific components, vehicle speed or conditions. Begin the road test with a general brake performance check. Using the description of the concern, test the brakes at different vehicle speeds using both light and heavy pedal pressure. Determine if the concern is in the front or rear braking system. First use the foot brake and then the parking brake. If the condition (i.e. pull, vibration, pulsation) occurs only on operation of the parking brake, the concern is in the rear brake system. If the condition occurs when the foot brake is depressed, the concern is in the front brake system. Avoid locking the brakes and sliding the tires. This condition will not indicate brake efficiency. A heavily braked but turning wheel will stop the vehicle in a shorter distance than locked wheels. If the concern becomes evident during this check, make sure it fits the description given before the road test. If the concern is not evident, attempt to duplicate the condition using the information from the description. If a concern exists, use the Brake System Symptom Chart in order to isolate it to a specific sub-system and condition description. From this description, a list of possible sources can be used to further narrow the cause to a specific component or condition.
Pinpoint Tests
Hydraulic Leak Check
NOTE: There is a common clutch and brake fluid reservoir,
therefore it is possible that a clutch leak can lead to reduction in the
reservoir level.
It is possible that all evidence of fluid leakage may have washed off if the
vehicle has been operated in rain or snow, as brake fluid is water-soluble.
Refill the system, bleed then apply the brakes several times. Examine the system
to verify that the reservoir fluid level is actually dropping. Locate and repair
the external leak. If the fluid level drops and no external leak can be found,
check for a brake master cylinder bore end seal leak.
Brake System Check
Brake Pedal Reserve Check
Where a low brake pedal or the feel of a bottomed-out condition exists, check
for brake pedal reserve.
NOTE: This increased resistance may feel like something has
bottomed out.
NOTE: The additional movement of the brake pedal is the result of
the increased engine manifold vacuum which exerts more force on the brake
booster during engine rundown. This means that additional stroke is available in
the brake master cylinder and the brake system is not bottoming out.
Brake Booster Functional Test
Inspect all hoses and connections. All unused vacuum connectors should be
capped. Hoses and their connections should be correctly secured and in good
condition with no holes and no collapsed areas. Inspect the check valve on the
brake booster for damage.
Brake Booster Operation Check
Brake Master Cylinder
Usually, the first and strongest indicator of anything wrong with the braking
system is a feeling through the brake pedal. In diagnosing the condition of the
brake master cylinder, check pedal feel as evidence of a brake concern. Check
for the red brake warning indicator illumination and the fluid level in the
master cylinder reservoir.
Normal Conditions
The following conditions are considered normal and are not indications that
the brake master cylinder is in need of service.
Abnormal Conditions
NOTE: Prior to performing any diagnosis, make sure the brake
system warning indicator is functional.
Changes in brake pedal feel or travel are indicators that something could be
wrong with the braking system. The diagnostic procedure and techniques use brake
pedal feel, warning indicator illumination and low brake fluid level as
indicators in diagnosing braking system concerns. The following conditions are
considered abnormal and indicate that the brake master cylinder is in need of
service.
Brake Master Cylinder - Compensator Port Test
Brake Master Cylinder - Bypass Condition Test
REFER to: Lifting (100-02 Jacking and Lifting, Description and Operation). Apply the brakes with a minimum of 445 N (100 lb) and hold for approximately 15 seconds. With the brakes still applied, exert a torque on the front wheels of 10.1 Nm (75 lb ft). If either wheel rotates, install a new brake master cylinder. REFER to: Brake Booster - Vehicles With: LHD FWD (206-07 Power Brake Actuation, Removal and Installation).
Non-Pressure Leaks
Any empty brake master cylinder reservoir condition may be caused by two
types of non-pressure external leaks.Type 1: An external leak may occur at the brake master cylinder reservoir cap because of incorrect positioning of the gasket and cap. Reposition the cap and gasket. Type 2: An external leak may occur at the brake master cylinder reservoir mounting seals. Service such a leak by installing new seals. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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