What is the problem when you can not steer nor brake the car 96 GMC safari its very hard t o steer the steering wheel and hard to brake its very hard when pressing the brake pedal?

There is a vacuum attachment on the power brake booster. If there is a vacuum leak this part could be causing the brakes to be hard, or this part could be the vacuum leak. The steering could also be vacuum assisted occurring from the same source.

Are there any other symptoms (i.e. vent/floor/defrost not blowing air or changing properly)?