Vacuum brake booster
1. Inspect all hoses and connections. All unused connectors of the vacuum system must be closed with plugs. Hoses and their connections must be properly secured, in good condition, free of holes and deformed areas. Inspect the brake booster check valve for damage.
2. Check the hydraulic brake system for leaks or low fluid levels.
3. Put the transmission in PARK (Automatic transmission), stop the engine and apply the parking brake. Apply the brake pedal several times to completely relieve the vacuum in the system.
4. With the engine turned off and there is no vacuum in the system, apply the brake pedal and hold it depressed. Start the engine. If the vacuum system is working, the brake pedal will tend to move down with constant foot pressure. If no movement is felt, the brake booster system is not functioning.
5. Remove the vacuum hose from the vacuum brake booster. Manifold vacuum must be present in the brake booster side hose with engine idling and transmission in PARK or NEUTRAL. Ensure that all unused vacuum connections are properly plugged, hose connections are properly secured, and vacuum hoses are in good condition. After checking for vacuum from the manifold to the brake booster, connect the vacuum hose to the brake booster and repeat step 3. If no downward movement of the pedal is felt, the brake booster system is not functioning.
6. Let the engine run at high idle for at least 10 seconds. Stop the engine and let the car sit still for 10 minutes. Then apply the brake pedal with a force of approximately 89 N. Pedal feel (applying the brakes) should be the same as when the engine is running. If there is a hard feeling when you press the brake pedal (no boost effect), install a new check valve, then recheck. If the brake pedal still feels tight, install a new brake booster. If there is a soft feeling when the brake pedal is moved ("sponges"), BLEED the brake system. Refer to section "General procedures".
Master brake cylinder
As a rule, the first and strongest sign of any malfunction in the brake system is the sensations perceived through the brake pedal. When diagnosing the condition of the brake master cylinder, check for the feeling that occurs when the brake pedal is applied, which confirms a malfunction in the brake system. Check the brake system warning light and the brake fluid level in the brake fluid reservoir.
Normal states
The following conditions are considered normal and do not indicate a need for brake master cylinder repair.
1. New brake systems are not designed for as much force applied to the brake pedal as before. Complaints of light pedal force should be verified by comparison with the pedal force of another vehicle of the same model and year.
2. During normal operation of the brake master cylinder, the brake fluid level in the reservoir rises when the brakes are applied and falls when the brakes are released. "Clean" brake fluid level (after applying and releasing the brakes) remains unchanged.
3. There are traces of brake fluid on the body of the vacuum brake booster, under the mounting flange of the main brake cylinder. This occurs as a result of normal lubrication of the mechanical seal bore of the master cylinder.
The brake fluid level will drop as the brake pads wear.
Abnormal conditions
Changes in how you feel when you press the brake pedal or when you move the brake pedal indicate a possible malfunction in the brake system. The procedure and diagnostic methods include the analysis of the sensations that occur when you press the brake pedal, the warning light of the brake system and the presence of a low level of brake fluid, which are signs of a possible malfunction in the diagnosis of complaints related to the brake system. The following conditions are considered abnormal and indicate the need for brake master cylinder repair.
1. Fast downstroke of the brake pedal. The reason for this may be external or internal leakage.
2. Slow down stroke of the brake pedal. The reason for this may be external or internal leakage.
3. Low drop or soft feel of the brake pedal. This condition may be due to a lack of fluid in the brake reservoir, a blocked vent in the brake reservoir cap, or air in the hydraulic system.
4. Excessive effort when applying the brake pedal. This condition may be due to a stuck or locked pedal or linkage, a blocked check valve, or insufficient vacuum in the brake booster.
5. Blocking the brakes of the rear wheels with a slight pressure on the brake pedal. This can be caused by grease or fluid on the brake pads, damaged brake pads and linings, improper parking brake adjustment, or a damaged or dirty brake pressure control valve.
6. Random change in effort on the brake pedal. This condition can be caused by a faulty brake booster, a brake caliper piston retracting in the end position, or incorrectly installed brake pads.
7. The control lamp of the brake system is always on. This condition may be due to low brake fluid, the ignition wire being too close to the low brake fluid warning light, or a damaged float.
Bypass test
1. Check the brake fluid level in the brake fluid reservoir. If the fluid level is low or the reservoir is empty, fill the reservoir with brake fluid up to the MAX mark.
2. Mark the brake fluid level in the brake fluid reservoir. If the brake fluid level remains unchanged after several applications of the brake pedal, measure the torque required to turn the wheels backwards with the brakes applied, proceeding as follows:
Place transmission in NEUTRAL position, raise and secure vehicle. See Section 100-02 for more information.
With the help of another technician, depress the brake pedal with a force of at least 445N and maintain the force for approximately 15 seconds. With the brakes applied, apply a torque of 10.1 Nm to the front wheels. If either wheel spins, INSTALL a new brake master cylinder.
Leaks without pressure
The absence of working fluid in the reservoir of the hydraulic drive of the brakes can be due to two types of external leaks in the absence of pressure.
Type 1: External leakage may occur in the area of the brake fluid reservoir cap due to improper gasket and cap position.
Reinstall gasket and cover.
Type 2: External leakage may occur at the sealing area of the brake reservoir mount. Repair such leakage by installing new seals.
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