Report "I have a brake pedal fading on a Nissan Sunny. It has power assisted brakes and no ABS. The pedal fades only with the engine running.It is hard and stays firm without engine power. On engine power it fades and stops hard slightly away from the floor board. I have replaced hoses, checked for leaks and plugged master cylinder. With plugged master cylinder the pedal stays firm on top even with engine on. Could it be the proportioning valve? What could it be. Thanks for any help"