Power brakes function only when theengine is running

Power brakes function only when theengine is running  - Brake system - Starting and driving - Volvo XC90 Owners Manual - Volvo XC90

The power brakes utilize vacuum pressure which is only created when the engine is running.

Never let the vehicle roll to a stop with the engine switched off.

If the power brakes are not working, considerably higher pressure will be required on the brake pedal to compensate for the lack of power assistance. This can happen for example when towing your vehicle or if the engine is switched off when the vehicle is rolling. The brake pedal feels harder than usual.

    See also:

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    The following audio and video files are supported by the system when playing a device connected to the USB socket. ...

    Deactivation
    Move the gear selector from R to another position. There is a slight delay in the PAC system, which means that the view from behind the vehicle will remain on the screen for approximately 15 se ...

    Operation
    Sport mode The stability system is always activated and cannot be switched off. However, the driver can select Sport mode, which offers more active driving characteristics. In this mode, the engin ...