Brakes on 2000 GMC Yukon XL 2500

My 2000 Yukon Xl 2500 brakes do not function properly. I've done everything like checked the brake pads and rotors, changed the brake line and changed the brake fluid and the brakes still do not work. Please help me?

Asked by for the 2000 GMC Yukon XL 2500
if you live in a northern climate, clean the speed sensors (on wheel hubs) brakes wont work if you are not getting any speed sensed (thinks you are allready stopped)....
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How about the master cylinder and calipers, because if these parts aren't performing you will not have brakes, Does the pedal slowly go to the floor?