Vacuum Brake Booster: Testing and Inspection
INSPECTIONOPERATION CHECK (WITHOUT GAUGES)
CAUTION: When checking operation, be sure to securely apply the parking brake.
^ CHECKING WITHOUT GAUGES
This method cannot determine the exact portion which has failed. But it can provide a rough understanding of the nature of failure if checking is conducted in accordance with the following procedures.
^ AIR TIGHTNESS CHECK
Start the engine, and idle it for 1 to 2 minutes, then turn it OFF. Depress the brake pedal several times applying the same pedal force as that used in ordinary braking operations. The pedal stroke should be greatest on the 1st depression, and it should become smaller with each successive depression. If no change occurs in the pedal height while in a depressed state, the brake booster is faulty.
NOTE:
^ In the event of defective operation, also inspect the condition of the check valve and vacuum hose.
^ Replace them if faulty and conduct the test again.
^ If no improvement is observed, check precisely with gauges.
^ OPERATION CHECK
1. With the engine OFF, depress the brake pedal several times applying the same pedal force and make sure that the pedal height does not vary with each depression of the pedal.
2. With the brake pedal depressed, start the engine.
3. As engine starts, the brake pedal should move slowly toward the floor. If no change occurs in the pedal height, the brake booster is faulty.
NOTE: If faulty, check precisely with gauges.
^ LOADED AIR TIGHTNESS CHECK
Depress the brake pedal while engine is running, and turn OFF the engine while the pedal is still depressed. Keep the pedal depressed for 30 seconds; if no change occurs in the pedal height, the brake booster is functioning normally; if the pedal height increases, it is faulty.
NOTE: If faulty, check precisely with gauges.
OPERATION CHECK (WITH GAUGES)
CAUTION: When checking operation, be sure to securely apply the parking brake.
^ CHECKING WITH GAUGES
Connect gauges as shown in the figure. After bleeding air from pressure gauges, proceed to each check.
^ AIR TIGHTNESS CHECK
1. Start the engine and keep it running until a vacuum of 66.7 kPa (500 mmHg, 19.69 inHg) = point A is indicated on vacuum gauge. Do not depress the brake pedal at this moment.
2. Stop the engine and watch the gauge. If the vacuum drop range is less than 3.3 kPa (25 mmHg, 0.98 inHg) within 15 seconds after stopping the engine, brake booster is functioning properly.
If defective, the cause may be one of those listed below.
^ Check valve malfunction
^ Leak from vacuum hose
^ Leak from shell jointed portion or stud bolt welded portion
^ Damaged diaphragm
^ Leak from valve body seal and bearing portion
^ Leak from plate and seal assembly portion
^ Leak from poppet valve assembly portion
^ LOADED AIR TIGHTNESS CHECK
1. Start the engine and depress the brake pedal with pedal force of 196 N (20 kgf, 44 lbs.). Keep the engine running until a vacuum of 66.7 kPa (500 mmHg, 19.69 inHg) = point B is indicated on vacuum gauge while the pedal is still depressed.
2. Stop the engine and watch vacuum gauge.
If the vacuum drop range is less than 3.3 kPa (25 mmHg, 0.98 inHg) within 15 seconds after stopping the engine, brake booster is functioning properly.
If defective, refer to "AIR TIGHTNESS CHECK".
3. If any fault is found on brake booster, replace the brake booster with a new one.
^ LACK OF BOOSTING ACTION CHECK
Turn OFF the engine, and set the vacuum gauge reading at "0". Then, check the fluid pressure when brake pedal is depressed. The pressure must be greater than the standard value listed.
^ BOOSTING ACTION CHECK
Set the vacuum gauge reading at 66.7 kPa (500 mmHg, 19.69 inHg) by running engine. Then, check the fluid pressure when brake pedal is depressed. The pressure must be greater than the standard value listed.