Hard to turning left

Two steering systems are listed for this truck. One uses a rack and pinion steering gear, and the other is the standard steering gear box. If you have a rack and pinion steering gear, turning hard to only one side is caused by internal leakage in the spool valve. The two clues are it always affects turning in only one direction at first, and it will return to normal effort after the power steering fluid warms up. That can take from less than a minute to fifteen to thirty minutes, and will take progressively longer and longer as more wear takes place. Eventually turning the other way will also be affected.

If you have the common steering gear box, it is much more rare, but a broken control valve lever can cause lack of power assist only one way. That will not be intermittent. The problem will never go away when the power steering fluid warms up.

The fix for either type of steering gear is to replace it. The clue that the power steering pump is okay is it is providing the needed pressure to turn in one direction. An alignment is required when replacing a rack and pinion assembly because the direction of steering must be adjusted for each front wheel. With the steering gear box, the alignment is only needed to insure the steering wheel is centered. Replacing this type of steering gear does not affect any of the other alignment angles or adjustments.

Aha. New observation and new valuable clue. I did not bring that up before because a weak power steering pump normally affects power assist both ways. When power assist comes back when increasing engine speed a little, it can be due to a weak pump. I am still concerned, though, with the hard steering just one way. You may still have excessive internal leakage in the steering gear box, but not a broken control valve lever. With a broken lever, you'd never get the power assist back, even when raising engine speed. With a little excessive internal leakage, raising engine speed could help the pump's higher volume overcome the loss of fluid pressure and give you back the power assist.

The point of my sad story is this one is going to be hard to know which unit is causing the problem, unless we can find someone who is aware of a common "pattern" failure. We never approve of throwing random parts at a problem in hopes one will fix it, and that is even more important here when either suspect is expensive. Since replacing either part is rather involved, and not one anyone would want to go through the work of putting the old part back on, a mechanic would perform a pressure test first. That involves installing a test gauge inline with the high-pressure hose between the pump and the steering gear. The unit also has a valve to block fluid flow.

Power steering pumps have an unusual valve that I do not fully understand, but it causes fluid to internally bypass the steering gear when you are not turning the steering wheel, and it allows fluid pressure to suddenly go very high when you do turn the steering wheel. For that reason, the readings on the gauge have to be taken quickly before pressure drops, so the mechanic has just one or two seconds to do so. The valve is tuned off to block fluid flow, then the highest pressure is read on the gauge. If it meets specs, which is typically around 1100 psi, it is assumed the pump can develop the correct pressure, but that does not say anything about supplying volume while maintaining that pressure. This is where you might get good power assist only while turning the steering wheel slowly, but not during rapid steering maneuvers.

If the specified pressure can never be achieved on the test gauge, the pump is suspect, but that does not totally rule out the steering gear box. A strong, new pump can overcome the leakage in a steering gear, making it look like the problem is solved, and it may be for many years. Likewise, installing a rebuilt steering gear with no internal leakage could make it unnecessary to replace a weak pump. Now you see the dilemma your mechanic goes through when he is trying to give you an accurate repair estimate.