Hydrauilic assisted direction problem

The steering on my Chevrolet truck is extremely stiff when turning. I've been told it's the power steering pump, but I don't know why, even though the oil seems to be circulating.
We've done everything the manual says to bleed the air from the pump outlet, the line going into the steering box, and the line coming out. The oil is coming out, but I have no way to test the pressure, and I'm still having the problem that when I'm driving slowly or trying to park, the steering is extremely stiff; it becomes completely manual, even though it's a power steering system.

The pump may be leaking oil, but not enough to open the valve

Exactly, but you should also check the condition of the oil and its pressure, as well as the tension of the belt.

Hello.
Do you have hydraulic or power steering?