Break system help?

[Papa Paul]

Hello.

I have a 1997 GMC Sierra 1/2 ton, full-bed pick-up. I've just installed a new MC, rotors, top-of-the-line pads, calipers, one new line to the rear, one WC in the rear and new rear shoes. Everything seems to be adjusted and there are no leaks. The system has been bled 4 times. I have by-passed the ABS which was not working...(don't need it?) The pedal is up full until you start the truck, then goes almost to the floor upon starting. We disconnected & plugged the MC and the pedal seems to be working with no leaks in the vacuum part on the fire wall. I am starting to think I have bought a faulty MC. Any thoughts and help would be GREATLY APPRECIATED! We are confused. Thanks for reading and have a good day. Paul.