Chevroleth 3500HD disc brake problem

Good afternoon I have a problem with the Chevroleth 3500HD brake system and I would like to know if someone can guide me to solve it the vehicle was made only change of front wheel drive at the end of assembling it was going to try and when the brake the brake is braked the unit of the four wheels without stepping on the pedal to free it we loosen the pothogue tied the four wheels we are purging the entire system and the problem persists