Please help diagnose power steering issue

The symptoms:

Steering has virtually zero power assistance at a stop and at idle. (Pump? Serpentine belt tension?) - Belt tension feels good, so it must be pump...

Driving around town the steering feels fine.

If I bring the RPM's up, power assistance starts again, although it's not perfect - another vote for pump.

Truck wanders on windy roads and the freeway (steering rack? pump?) - Steering rack has had its fair share of abuse (submerging, mud, the long travel yanking on it, etc.)

I've flushed the system twice and bled it repeatedly each time.


I can't decide between the pump and the rack... I used to always think it was the rack but bringing the RPM's up and steering coming back makes me think it could be the pump.

The serpentine belt is tight, but is old. Not that much in the way of cracks on it.

Ideas?

 

Additional info:

I'm working solo, so observing the serpentine belt while turning the steering wheel takes creativity. I tied a rope around the steering wheel and turned it as I watched the engine compartment.

The groan is definitely coming from the pump. It does not appear the belt is slipping.