Sorry for taking so long, with work I really don't get much time to work on the Z, but here's some updates.
I replaced the MC with a new one that came with the car. I properly bench bled it.
Properly bled each brake line with someone pressing the pedal. I would have them hold the pedal down, open the bleeder, close the bleeder, let off the pedal, repeat.
I've found out that the block with the "Brake Line Pressure Differential Warning Light Switch" is leaking, but only when force is applied to the brakes. seems like it's leaking from one of the two ports on the bottom where the MC connects.
When I start the car, the brake pedal goes straight to the floor, but if I pump it, it builds pressure and gets firmer. If I let it run for a a couple seconds after that all pressure is lost and the pedal goes straight to the floor.
Also found a plug unplugged and the "brake warning" light on the dash now comes on when I press the brakes and the pedal goes to the floor. When I pump them and the pedal gets stiffer, the light doesn't come on.