With steering it is pretty easy to say what makes it power. There is a belt driven pump that drives hydraulic fluid into a valve that turns the wheels. With brakes, on the C3 cars, there is a vacuum assist. It is the "swishing" sound you sometimes hear when you press the brake pedal. These systems lessen the amount of "human" force necessary to actuate the systems.
With steering the difference between power and non-power steering is most apparent at slow (parking lot maneuvering) speeds. It sometimes takes quite a bit of arm muscle to move the steering wheel to make the wheels turn with non-power steering. It is always easier to turn the steering wheel when the car is moving, even slowly, than when the car is stationary.
With the brakes, the brake booster increases the amount of force applied through the brake lines which in-turn increases the pressure in the caliper.
The problems you might run into with power is more lines to leak in the steering. With the brakes, a vacuum leak does wonders for emissions if you have to be tested. Also, power brakes and steering are very difficult to use if your cars engine dies while you are moving. The effort required to actually steer the car and brake the car increase significantly with no power assist. It happened to me once while I was in my '78 going downhill in the Smoky Mountains! (ignition coil died, 2-lane road, no shoulder) With non-power systems, you do not have to worry about power loss. The systems will still work even if the engine dies.
Hope this helps.
