The tandem master cylinder braking system is a hydraulic system with two separate subsystems. If one of the subsystems fails, the second one will still work. However, the pedal will become harder to press and your braking distance will increase. The brake warning light may also come on.
Brake booster
The brake booster uses engine vacuum to increase braking power. If the engine stalls while driving, you can stop the vehicle by simply depressing the pedal. The engine vacuum reserve is enough for one or two stops - but no more!