Brake pedal goes to the floor on first pump

I have replaced everything brake related on this vehicle. The only thing that I haven’t is the hard lines on the body.
What is happening is on the first push of the brake pedal it goes right to the floor. You can then pump it up and it will hold. It will not fall. There is no leak from anyway. I have sent it to a shop that has adjusted the push rod and installed another new master. They are having the same problem. They can not find a leak ether. They performed a pressure test and still no fault found. They are at a point to start throwing parts at it. But I would rather have the problem diagnose then just throwing parts at it. My question is, is there some other ways to try and diagnose or repair this first pump pedal falling?