common problem..
this is a common problem for a CUMMINS powered trucks. our power brakes and power steering use the same pump for the power assist (hydralic over hydralic braking) after the pumps are worn a little they do not produce the gallons per minute or pressure at an idle that they used to (should). some people have had a little luck switching to synethic fluid, others have taken the pump apart and put a washer in the pressure regulator spring, thus increasing the pressure a little bit. this seems to have gotten good results... but it does increase your line pressure every where, thus maybe creating leaking problems down the road.
the best and easy fix is don't ever turn the wheels while sitting still on any vehicle... this is so hard on the steering componets with all of that static weight on the steering axle. it will also lead to premature wear and a sloppy steering box, not to mention all the tie rod and pitman ends and such. the other thing is if you must turn the wheels sitting still, do not apply the brake and or rev the engine slightly to improve the GPM and PSI.
hope this helps ya out