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 normal pedal force. The pedal stroke should be the longest at the 1st depression, and it should become shorter at each successive depression. If the depressed pedal height is not changed, the brake booster is faulty.

Fig 1: Depressing Brake Pedal

Courtesy of SUBARU OF AMERICA, INC.

NOTE:

In case of defective operation, inspect the condition of the check valve and vacuum hose as well.
Replace them if faulty, and perform the test again.
If a defect is not improved, check correctly using a gauge.