Power steering issue

I have very stiff power steering in both directions at low speeds or not moving. Over about 10 mph it steers nicely. The hoses are new and in good shape and the filter is also new, with new 15/40 oil to the correct level. I do not have a flow meter but I did tee in a pressure gauge. When I jack up the front end the gauge shows about 400 psi, and then goes to zero when hitting the steering stops. With the wheels on the ground the gauge shows only 500 pounds. Is there possibly a relief valve somewhere (Cummins ISM gear driven pump) , or is the pump bad, or is there something else I am missing? Seems like it should get to 1000 or more to overcome steering effort. I do not see a relief on the pump or the box, but I may have missed it.
Thanks for the help.

 

I cannot turn the wheel from center while sitting still. When I raise the RPM the pressure moves up to about 600 and the wheels turn with difficulty. Seems like it is already deadheading at about 600, and needing 800 or 900 to turn while not driving. This truck is not on the road yet so I cannot drop it at a steering shop and they decline to rent me a flow meter, but they did say it sounded like a bad pump.

 

I will try hooking the gauge directly to pump pressure.

 

Just to close this off, it was the pump. It only made 500 at idle deadheaded so I replaced it and now I am back to one finger turns.