Nissan Pathfinder Brake problem

Ok this is stumping me AND my mechanics. Maybe someone cane come up with some new ideas.

Ok so the vehicle is a 2000 Nissan Pathfinder. Anyone who has owned one seems to know the brakes are a little softer than other vehicles.

So about two weeks ago the brakes started going softer and softer to the point that the brake pedal needed to go down to the floor before the brakes would start to work.

I checked the fluid myself, everything was fine, to the top. Even bled the brakes, no problems.

So I took it into my mechanics and at the end of the day (after making sure all my brake pads, rotors, drums, etc were fine) they told me that my master cylinder was probably acting up. They fired in a new one, bled the brakes, and took it for a test drive. Still had problems.

They tested the ABS sensor and it was bad. So they went and threw a new one in there. Took it for a test drive, still had brake problems.

What is happening is the brake is going right to the floor and then you have to let it off and press the brake again. The second time the brakes work like they should.

So they tried bleed the brakes again. Didn't help. They fired off a phone call to Nissan themselves. They said to try bleeding the brakes a different way. So they did. Still no help.

They checked all the flex lines on the brakes to make sure the lines were bulging anywhere. They were fine.

Checked all the brake cylinders. They were fine. Tried to tighten all the brakes. They were good.

They are literally stumped. The only thing he hasn't played with is what appears to be a "load sensing proportioning valve" or "load sensing valve". Now the mechanic isn't really sure exactly how this thing operates. He tried search online but couldn't really figure out how it mechanically works on a Nissan. And it's an enclosed unit so he can't really open it up to find out. It has two brake lines going in and one coming out.