Brake booster
The brake booster uses engine manifold vacuum to assist braking force. Do not turn off the engine while driving because that will turn off the brake booster, resulting in poor braking power.
The brakes will continue to work even when the brake booster completely stops functioning. If this happens, however, you will have to push the pedal much harder than normal and the braking distance will increase.
See also:
Precautions against vehicle modification
To avoid accidental activation of the system or rendering the system inoperative, which may result in serious injury, no modifications should be made to any components or wiring of the SRS airbag ...
Adjustable level of each volume
...
One Gear Shy
To solve the problem for good, and improve the car markedly, Subaru needs to
replace the five-speed manual with a six-speed. So that you know where I'm
coming from, I'm not someone who demands six ...