Brake pedal slowly goes to floor

1986 535i

Your first thought will be Master cylinder. I replaced it with a used part off another car, still had the same issue, then bought a new one and it hasn't changed the problem. (300$ down the drain)

Symptoms:
With the car running the pedal slowly, with substantial force, creeps to the floor.

I do get braking power, but not 100%.

With the car off, they are hard.

I have pressure bled them several times. 2-3

The fluid level does not drop, even after putting the pedal to the floor 20 times. So no leaks.

We thought maybe there was a flex line that was flexing but I could not feel any bubbles while the pedal was pressed. I will attempt to pinch them with some Vise-Grips on the car side to double check.

Questions:

My manual does not specify a bleeding method for the MC. Is there one? Could this be the issue even after having pressure bled the system?

Could my Hydro boost be applying too much pressure to the MC and overcoming the seals? Otherwise I can't see how this system could affect it.

Any other thoughts?

I am desperate as I need this car on the road ASAP.

That usually indicates air in the system. Bleed the brake fluid from each wheel with a clear piece of tubing into a jar so you can clearly see when the bubbles stop. You will probably have to go around the car a few times. Start with the RR, LR, RF and lastly LR. Leaving the bleed screws open overnight to "gravity bleed" the system might also be helpful.