Lost brakes out of the blue?

[DylanM240]

So isn't on the road right now but is operational, it hasn't sat(engine was swapped over winter) after my swap was done the car was driveable, I had brakes and everything. I took my car out of the garage today and I have zero brakes, to the floor. I have checked the following.

-Brake booster holds vaccum(as I took my line off of the check valve it released pressure)
-Brakes pressurize when car is off(Master cylinder is still good)
-Bled brakes today, all four corners and none even had air in them to begin with.
-When I depress pedal and turn engine over, pedal lets in.

I honestly can't think of what is causing this? I'm very stumped.

[Biggins240sx]

I would pull the master cylinder off just to be sure it is not leaking into the booster. Can't hurt it by trying. Also make sure none of the calipers are leaking from the piston. Had one do that to me.