2021 Subaru Forester Owner’s Manual — Brake booster

The brake booster utilizes the vacuum produced by the engine manifold to reduce the force required to depress the brake pedal.

The vacuum in the brake booster may be insufficient if the brake pedal is depressed when the engine is stopped or after the vehicle has been parked for a long period of time. It such cases, it is necessary to depress the brake pedal using greater force than usual.