I have a 1999 GMC Suburban 1500. The problem I am having is when I press on the brake pedal all I hear is air like it has air brakes or something and the brakes barely stop when idling or in reverse. When the truck is off it doesnt make the air sound when I press the pedal. I have bled the brakes and it has plenty of fluid. The brake pads have at least have life and there is no air in the brake lines. Help Please!