Operation check

While the engine is OFF, depress the brake pedal several times applying the same pedal force, to check for a change in pedal height.

Fig 1: Checking Brake Pedal Operation
Courtesy of SUBARU OF AMERICA, INC.
With the brake pedal pressed, start the engine.
As the engine starts, the brake pedal should move slowly toward the floor. If the pedal height does not change, the brake booster is faulty.
NOTE: If faulty, perform an accurate check with a gauge.