I have a 2006 GMC sierra Denali 1500 with a 6.0L. I bought the truck in July 2014. It currently has 80,615 miles on it. The truck has very spongy brakes. Has since I got it. At Christmas new pads and rotors were put on all the way around. I was told to test master cylinder by pumping the brakes without the key on. The pedal gets very firm that way. While truck is running I can pump the brakes and they stay spongy and if I hold the brakes I can push pedal to the floor. I was also told it was possible the ABS sensor on the hubs might be bad so to pull the ABS fuse and drive like that. So I did that and there is no change. Im not sure where to go from here. Any help would be greatly appreciated.