Air Tightness Check

Start the engine and keep it running until vacuum pressure indicates A point of vacuum gauge = 66.7 kPa (500 mmHg, 19.69 inHg). Do not depress the brake pedal at this time.

Fig 1: Identifying Pressure Gauge And Vacuum Gauge
Courtesy of SUBARU OF AMERICA, INC.
Stop the engine and check the gauge. If the vacuum pressure drop within 15 seconds after stopping the engine is less than 3.3 kPa (25 mmHg, 0.98 inHg), the function of brake booster is normal. If faulty, the cause may be one of the following.
- Check valve malfunction
- Leak from vacuum hose
- Leak from shell joint section or stud bolt welded section
- Damaged diaphragm plate
- Leak from valve body seal and bearing section
- Leak from plate and seal assembly section
- Leak from poppet valve assembly section