The brake booster delivers additional force to the brakes and dramatically decreases stopping distance of Your vehicle.
This could be very dangerous when brake booster is broken. Here are some symptoms of a failing brake booster:
The Brake Pedal Is Hard To Press - if You can't control your brakes then You can't control speed of Your vehicle.
Longer Braking Distance - probably there's an air in Your brake system.
Higher Brake Pedal - if you feel that your brake pedal became higher probably there's a problems with your brake booster.
Hissing Sound Under The Dash - probably by the escaping vacuum.