Changed brakes and rotors on2005 GMC ENVOY, THE PEDAL is hard when vehicle is not running, but is soft when running

SOURCE: changed brakes and rotors on 2005GMC Envoy,now brake pedal is hard when not running and soft when running?

This is the usual behavior. When the engine is running, it is generating a vacuum, which pulls on a diaphragm in the brake booster. This is what is called "power brakes". It uses engine vacuum to help push hydraulic fluid thus reducing the amount of pressure you must apply to the brake pedal. When the engine is off your foot has to make up for the force that engine vacuum usually applies, which makes the pedal feel much more firm.