i just replaced all pads and rotors on my 05 3500 4x4 dually diesel as well as the master cylinder. i bench bled the cylinder and well as bled the lines many many many times but the pedal still goes almost to the floor before the brakes start engaging. question is, is there something im doing wrong. the pedal seems like it is holding pressure when the truck is off, but as soon as i start it holding the pedal, it falls to the floor. any ideas would be helpful at this point! thanks, steve

 

The best results I've had with a stubborn air pocket in a brake system is to use a pressure bleeder backwards starting at the furthest bleeder valve from the master cylinder and working forwards to the closest...Pressurizing the calipers/wheel cylinders forces air bubbles to rise to the MC reservoir and out of the system......Ben

 

Crack all the bleeders and let it gravity bleed. I'm guessing as a result of changing the master, you got air trapped in the ABS pump.