Vacuum brake booster
1. Inspect all hoses and connections. All unused vacuum system connectors should be capped. Hoses and their connections should be properly secured, in good condition, and 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. Place the transmission in PARK (automatic transmission), turn off 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 off and no vacuum in the system, apply and hold the brake pedal. Start the engine. If the vacuum system is working, the brake pedal will tend to move downward with constant foot pressure. If no movement is felt, the brake booster system is not functioning.
5. Remove the vacuum hose from the brake booster. Manifold vacuum should be present in the hose on the brake booster side with the engine idling and the transmission in PARK or NEUTRAL. Make sure all unused vacuum connections are properly capped, hose connections are properly secured, and vacuum hoses are in good condition. After verifying that manifold vacuum is present to the brake booster, reconnect the vacuum hose to the brake booster and repeat step 3. If downward pedal movement is not felt, the brake booster system is not functioning.
6. Run the engine at high idle for at least 10 seconds. Stop the engine and let the vehicle stand still for 10 minutes. Then apply the brake pedal with a force of approximately 89 N. The pedal depression (brake application) should feel the same as when the engine is running. If the brake pedal feels hard (no boost effect) when depressed, install a new check valve, then recheck. If the brake pedal still feels hard, install a new brake booster. If the brake pedal feels soft (spongy), BLEED the brake system. Refer to the General Procedures section.
Master brake cylinder
Typically, the first and strongest indication of any problem with the brake system is the sensation felt through the brake pedal. When diagnosing the condition of the master cylinder, check the sensation that occurs when the brake pedal is applied, which confirms a problem with the brake system. Check the brake system warning light and the brake fluid level in the brake reservoir.
Normal states
The following conditions are considered normal and do not indicate the need for master cylinder repair.
1. New brake systems are not designed to handle as much force on the brake pedal as in the past. Complaints about light pedal force should be verified by comparing it to another vehicle of the same model and year.
2. During normal operation of the master cylinder, the brake fluid level in the reservoir rises when the brakes are applied and falls when the brakes are released. The "clean" brake fluid level (after applying and releasing the brakes) remains unchanged.
3. There are traces of brake fluid on the brake booster housing, under the mounting flange of the master brake cylinder. This occurs as a result of normal lubrication of the end seal of the master brake cylinder opening.
The brake fluid level will drop as the brake pads wear.
Abnormal conditions
Changes in the feel of the brake pedal or the brake pedal stroke indicate a possible malfunction in the brake system. The diagnostic procedure and methods include analyzing the feel of the brake pedal, the illumination of the brake system warning light, and the presence of a low brake fluid level, which are all signs of a possible malfunction when diagnosing complaints related to the brake system. The following conditions are considered abnormal and indicate the need for repair of the brake master cylinder.
1. Rapid downward movement of the brake pedal. This may be caused by an external or internal leak.
2. Slow downward movement of the brake pedal. This may be caused by an external or internal leak.
3. Low brake pedal drop or soft feeling. This condition may be caused by a lack of brake fluid in the brake fluid reservoir, clogged vents in the brake fluid reservoir cap, or air in the hydraulic system.
4. Excessive effort when applying the brake pedal. This condition may be caused by a sticking or locked pedal or linkage, a blocked check valve, or insufficient vacuum in the brake booster.
5. Locking of the rear brakes when the brake pedal is pressed lightly. This may be caused by the presence of grease or working fluid on the brake pads, damage to the brake pads and linings, improper adjustment of the parking brake, damage or contamination of the brake pressure adjustment valve.
6. Erratic change in brake pedal force. This condition may be caused by a faulty brake booster, brake caliper piston retraction at the extreme position, or improperly installed brake pads.
7. The brake system warning light is always on. This condition may be caused by low brake fluid level, ignition wire running too close to the low brake fluid warning light, or a damaged float.
Bypass check
1. Check the brake fluid level in the brake hydraulic reservoir. If the fluid level is low or the reservoir is empty, fill the reservoir with brake fluid up to the MAX mark.
2. Note the brake fluid level in the brake fluid reservoir. If the brake fluid level remains unchanged after several brake pedal presses, measure the torque required to turn the wheels backwards with the brakes applied, as follows:
Place the transmission in NEUTRAL, raise and support the vehicle. For additional information, refer to Section 100-02.
With the assistance of another technician, apply a minimum of 445N of force to the brake pedal and maintain this force for approximately 15 seconds. With the brakes applied, apply 10.1Nm of torque to the front wheels. If any wheel rotates, INSTALL a new brake master cylinder.
Leaks without pressure
The absence of working fluid in the brake hydraulic drive reservoir can be caused by two types of external leaks in the absence of pressure.
Type 1: External leakage may occur in the brake fluid reservoir cap area due to incorrect positioning of the gasket and cap.
Reinstall the gasket and cover.
Type 2: An external leak may occur at the sealing area of the brake fluid reservoir mount. Fix this leak by installing new seals.
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