You didn't say how high the pedal is with the engine off. You can pump it several time to bleed off the existing vacuum. Then apply pressure to the pedal and start the motor. The pedal should drop a bit with the power assist (vacuum) but not go to the floor

I'm going to say master is bypassing internally or you have an external leak somewhere in the system. Check the fluid level as that may indicate which 1/2 of the system if it is a leak.
The pedal should not go to the floor unless there is a loss of pressure in the system