Operation Check (Without Gauges)

CAUTION: When checking operation, be sure to apply the parking brake securely.

Check without gauges

This method can not determine exactly what part is defective. But it is possible to identify the general type of defect if the following inspection is performed.

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 no change occurs in the pedal height when pressed, the brake booster is faulty.

Fig 1: Depressing Brake Pedal (Engine Running)

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 the defective condition is not improved, check accurately using a gauge.

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 2: Depressing Brake Pedal (Engine Off)
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.

Loaded air tightness check

Depress the brake pedal while the engine is running, and turn the engine OFF while the pedal is depressed. Keep the pedal depressed for 30 seconds. If the pedal height does not change, the function of brake booster is normal. If the pedal height increases, it is faulty.

NOTE: If faulty, perform an accurate check with a gauge.