Brake Servo System

With the engine off, depress the brake pedal several times applying the same pedal force. Check that the travel distance should not change.
With the brake pedal depressed, start the engine. Check that the pedal moves slightly toward the floor.
With the brake pedal depressed, stop the engine and keep the pedal depressed for 30 seconds. Check that the pedal height does not change.
A check valve is built into the brake booster nipple. Disconnect the vacuum hose to inspect function of check valve.
Check that check valve ventilates from booster side to engine side. Also, check that there is no ventilation from engine side to booster side.

Fig 1: Checking Check Valve Ventilates From Booster Side
Courtesy of SUBARU OF AMERICA, INC.
Check the vacuum hose for cracks or other damage.
CAUTION: When installing the vacuum hose on the engine and brake booster, do not use soapy water or lubricating oil on their connections.
Check that the vacuum hose is securely tightened.