Truck will not turn left but it turns right just fine

Sounds like the valve control lever is broken inside the steering gearbox. Since you have good power assist to the right, we know the pump has to be okay. That lever was actually adjustable on older models from the 1960's and 1970's, but everything is internal now. Plus, that adjustment was set only once to its proper adjustment. It was not something that needed to be fiddled with for the rest of the life of the truck.

When you turn the steering wheel, what you are doing is opening a valve that lets pressurized power steering fluid enter and push on one side of the power piston in the gearbox. As that piston moves, it pulls the valve closed again, unless you continue to turn the steering wheel further. It is only when that power assist does not show up that parts link together mechanically as a back-up so you can never lose steering control. That is what you are feeling now when you turn to the left.

To double-check my suspicion, jack the front end off the ground, then run the engine to verify you still do not have power assist to the left. If you do now, there is an external mechanical problem in the form of a damaged or bent steering linkage.

Also, with the engine off and the tires still off the ground, you should feel equal resistance when turning the steering wheel both ways. Power assist is totally out of the picture now. If both directions feel the same, suspect the gearbox. If you hear or feel any roughness or binding, check the steering linkages for bent parts.

An internal problem with the gearbox is very rare, but equally uncommon is a problem with the steering damper. That looks like a shock absorber connected to the axle housing on one end, and the steering linkage on the other end. It is supposed to pull apart and push together equally hard either way, and it does need some force to do that. If you see its housing is dented or the shaft is bent, that can also cause hard steering one way unrelated to the gearbox or power steering system. If it is stained with oil that has leaked out, it likely has come apart inside and is binding. As a test, you can disconnect one end of it, then see if the steering is back to normal. If you drive the truck that way, be sure the damper is tied up so it does not hit the ground or interfere with the steering linkage. Do not hit highway speeds either. Trucks tend to develop what we refer to as the "death wobble". That is when the steering wheel shakes violently back and forth at higher speeds. You will never forget the first time you experience that! That is what the damper is there to prevent.

You might get more ideas from this article:

https://www.2carpros.com/articles/power-steering-problems

Very doubtful. You must remember the symptom only affects turning one way, so we have to consider only things that both directions do not have in common. If low pump pressure, contaminated fluid, or even a loose drive belt were the cause, it would affect turning both ways.

The pump is the only place where heat is generated in the power steering fluid, so if that fluid was dark, it is in the pump where that occurred. There are sliding vanes in it that can wear down and result in reduced fluid pressure, and there is a valve in the outlet port that reduces fluid pressure when you are not turning, but again, both of those are not related to the direction you are turning. The only thing I can think of is if some debris broke off one of the vanes, (they are just a fiber material), it might have circulated through the system and lodged in the steering gear box. The power piston has steel sealing rings on it and those are not going to be damaged by debris. What I suppose could happen is a port became blocked and will not let pressurized fluid through to do its thing, but in doing steering, suspension, and alignment repairs for over thirty years, I have never solved this symptom by replacing a gear box. This is a real common problem with rack and pinion steering gears, especially on older GM front-wheel-drive cars, but while it can happen on any brand, I have never seen it with the "recirculating ball: gear box design that you have.

Also, the left and right-turn ports in the gear box are shared. When turning right, fluid goes in one port, then out the other one. Their roles are reversed when turning the other way. That means turning one way should blow out any blockage and send it back up the pump's reservoir.

There is one more thing you might look at. That is the round or hexagon-shaped cover on the gear box right where the steering shaft goes into it. If someone recently tried to replace that seal or did some other repair, and left that cover less than fully-tightened, it will allow the control valve lever to move back and forth when the steering wheel is turned, but it will not open that valve one of the two ways. This is something that is not going to happen on its own unless that lever cracked, or unless someone had it apart recently. The way to identify that is to watch that stub shaft and/or steering coupler while a helper turns the steering wheel back and forth. Do that with the engine running, and only move the steering wheel about 1/8th turn back and forth, about twice per second. If something is wrong there, you will see the stub shaft and coupler move away from and closer to the gear box. That in-and-out movement should be small enough to not be perceptible. There are three things on the four-wheel-drive models of these trucks that can cause objectionable steering wander at highway speeds. In-and-out movement of that shaft is by far very uncommon, but it is one of those three things to look for.

To check for a blocked port is more involved than most independent shops can do. Even most dealership mechanics will not be able to find the test gauge for this because it gets used so seldom. It is an inline pressure gauge that is connected in the high-pressure supply hose. When you turn the steering wheel, it opens that valve in the gear box, then fluid flows in and pushes on the power piston. That flow through the pump's outlet port signals the pressure relief valve to increase fluid pressure up to as much as 1,100 pounds. If the pressure remains low when turning left, it suggests the port is blocked in the gear box or the control lever is cracked. If pressure goes very high, but you still do not feel the power assist, it is because the gear box is having difficulty moving the steering linkage, but it is trying to. That is where the steering damper or bent linkage are the better suspects. If the linkage is binding, you should hear and feel that. Also, that would be affected by the steering wheel position, not the direction of travel. For example, if a linkage rubs on something when the wheels are turned fully-left, you will have hard steering both ways at that time, and easy steering both ways when turned fully to the right.

There is a chance there is a problem inside the steering gear box, but before you rush to replace it, a better approach would be to have the steering and suspension systems inspected at a tire and alignment shop. Many of them offer these inspections at no charge in hopes you will let them repair what they find.

These gear boxes are not something a mechanic would disassemble to repair. They are too complicated and often expensive special tools are needed. I have had a few apart to replace seals, and to see what made one fail, but I would never take on the liability with a customer's vehicle. Repair parts generally are not available either. This is why you can only buy a used one from a salvage yard or a professionally-rebuilt one with a warranty. These had another problem with the bushing the pitman shaft rides on where it comes out the bottom of the housing. Wear to that bushing allows the entire steering linkage to move very slightly resulting in annoying steering wander, and eventually a fluid leak when that wear gets bad enough. For this reason, a rebuilt gear box is the better alternative over a used one.