brake pedal goes to the floor at start up

Did you buy a new or rebuilt M/C unit?

You can isolate the M/C by clamping off the 3 brake hoses using rags or towels in the jaws of some vise grips so as not to damage the hoses. If the pedal drops w/ the hoses clamped closed, it's the master most of the time.......

Best I can do to help. Good luck, brakes can sometimes be a PITA.