Loaded Air Tightness Check

Start the engine and depress the brake pedal with a pedal force of 196 N (20 kgf, 44 lb). Keep the engine idling until vacuum pressure indicates B point of gauge = 66.7 kPa (500 mmHg, 19.69 inHg) with the pedal depressed.

Fig 1: Identifying Pressure Gauge, Vacuum Gauge And Pedal Force Gauge
Courtesy of SUBARU OF AMERICA, INC.
Stop the engine and check the vacuum 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 defective, refer to "AIR TIGHTNESS CHECK". <Ref. to INSPECTION , Brake Booster. >
If the brake booster is faulty, replace it with a new part.