If the engine stops running, or if the primary brake system stops working, your vehicle has a reserve power assist system to help you slow down. Just slowly and steadily apply the brake pedal until you can safely get off the road. The pedal will seem harder to push down. Do not pump the pedal; the system will not work well that way.
You might find that the steering wheel seems hard to turn when you are turning and braking at the same time. Also, the primary brake warning light might come on and the warning tone might sound. This is normal because the main hydraulic brake system and power steering both use the power steering pump. If this ever happens, let up on the brake pedal a little. When you let up on the brake pedal in that situation, it lets the steering get a little more help from the pump.