Power Steering Weird Problem

[Wolfe440]

Hi,
71 Charger 440.

Got a weird problem with the power steering, nothing I have ever experienced over decades of my many Mopars.

The issue I am having is when I start the car cold the power steering works fine. I swing the steering wheel back and forth with good power steering, but after about two minutes it begins to give me resistance both ways until there is no power steering assistance at all regardless of the rpm. I shut off the car and start it again and no power steering. If I leave it sit overnight it only repeats the same results the next morning.

The fan belt is new and plenty tight and not slipping, the pulley is good and turns straight and true.
The original steering box from the car was sent to FirmFeel in 2017, its been in its box on my bench waiting for the point of the cars restoration to be installed and was two weeks ago.
It has bran new pressure and return lines that have been checked for any blockages and all clear. I have changed the steering pump THREE times, and I also took a well tested pump off another car that never gave me any problems and get the same darn results.
I also tried a new return valve on the top of the power steering box that the return line connects to and I still get the same results.   

I called FirmFeel, they say they cannot imagine any situation that the box would cause such a problem, they could maybe see having power assist turning one way and no assist the other, but not both ways losing power assist in the way I described. They didn't seem to be really helpful.

Any suggestions other then trying another steering box?

Thanks,
Wolf

[Wolfe440]

Just spoke to FirmFeel again, I am to send the steering box to them for bench testing. Hopefully it will offer a fix.