Here's what I wrote about 15 years ago when I had a 4104. Maybe it contains other information that help.
I bought a used Sheppard gear from a transit bus. It work OK except it had over an inch of free play. I purchased the factory service manual and it said up to 2 inches was normal. I just put up with it then became normal to me. It wasn't too bad except in a gusting side wind or on a section of pavement where the trucks had left ruts in the pavement (which is something you would never notice unless you had excess free play in your steering).
One day I decided it would be a good ideal if I let my wife drive the bus just in case she had to drive it for some reason. I've never seen anyone have such a hard time driving in a straight line. After watching her fight this thing for a few miles I realized that I had learned over the last few years how to drive with the excess free play. I knew at that point I had to fix this problem.
I call the most reputable steering gear rebuilder I could find. They promised me it would drive just like my ½ ton truck. I couldn't wait until the gear came back. It would be well worth the $1000 bucks that I spent but didn't really have. I installed it and the first thing I noticed was it felt harder to steer at idle. The test drive was disappointing, the free play was a little worse and it required more effort to steer when setting still. I called the rebuilder, they (let's call them Bill) lead me through a series of tests where I learned a few things but did not resolve the heavy feel at idle or the free play. Bill agreed to warranty the gear if I would play the freight, which was about $300 dollars. The gear came back, I reinstalled it and of course it was no better. Bill claimed it was full of trash. I had already cut the filter apart drained the contents of the filter and oil reservoir on a white cloth and there was no trash. At this point I knew that Bill didn't know much more than I did and he was not going to live up to his end of the deal.
At this point I had spent a little less than $2000 and had a gear that was worse than the one I got from the scrap yard in the first place. Part of that $2000 was for a flow meter that I purchased from Sheppard. I've forgotten the exact procedure, but with the flow meter you can measure the amount oil that is going by the piston, which indicates piston or bore wear. The gear that I paid good money to have rebuilt was bypassing slightly more than the max specification. Another test it failed was the amount of torque required to move the wheels at idle on pavement. I remember this test, make sure the front tires are properly inflated and that the vehicle is on level pavement or concrete. Next you'll need a torque wrench that will read up to about 200 inch pounds. Remove the horn button and use the necessary socket allowing you to measure the minimum torque required to move the front wheels with the engine at idle speed, you'll have to move the wrench very slow in order to get the most accurate reading. The rebuilt gear measured about 110 inch pounds. 100 inch pounds is the maximum specification and I think most people would say about 80 inch pounds should be the max specification.
Being hard headed and determined (is there a difference?) I was past the point of no return. I had to find a way to repair my ill driving coach. I knew it would take awhile to figure this out. I pulled the newly rebuilt gear off and set it aside. I when back to the place with the salvage buses and picked out another Sheppard gear, one of the tightest feeling gears out of a group of gears sitting on a pallet. I installed the used gear and rechecked the torque, it was 80 inch pounds and the free play was a little better than any of gears I had installed before. But it was still too much and the coach still did not drive like my truck or the MCI 102A3 that I owned for a short period of time. My goal was to make it drive similar to the MCI. I considered buying a new Sheppard gear so I called the Sheppard gear company and questioned them about free play. There is no way to adjust, shim or reduce the freeplay in that series of Sheppard gears. According to Sheppard and the Sheppard manual there can be up to 2 inches of free-play at the steering wheel rim even with a new gear. In my opinion the design is very durable, but more suited for off road heavy equipment. The very things that make it durable, no rings or seals on the piston and no way to adjust the free-play make it undesirable for a highway application.
I have done many alignments on automobiles and I know the effects that caster, camber, toe-in and king pin inclination have on steering, but I have to admit that I did learn some things when going through the checks that Bill required me to do.
One thing I had not considered is how important “return to center” is. For example in a parking lot, drive your automobile very slowly and turn the wheel sharply to the left or right then let go of it. The steering wheel will return to the center position very rapidly. Your bus should behave in almost the same manner. If it doesn't it will never drive correctly. As alignments go the caster and kingpin inclination have the most effect on this characteristic. On a straight axle you can't do much with KPI, some positive caster can be added to help with “return to center”. Another thing to check is the condition of of the king pins, if they are worn or even new and installed to tight this will affect return to center. The last cause is the one that I suspect most people never consider. How much drag or how much rotational force is required to turn the shaft that goes from the steering gear back to the steering wheel. The reason this is critical is because when you start turning your steering wheel it only takes about 6 inch pounds of force to start activating the hydraulic valving in the steering gear. That hydraulic pressure is the force that helps you turn the wheels. If your system has more than 6 inch pounds of rotational force from the gear back to the steering wheel your wheels will not return to center because the assist valving is starting to open, holding the wheels in whatever position they are in.
My bus had about 15 inch pounds from the gear back to the steering wheel. On my 4104 and most coaches with axle mount gears there is 4 universal joints, a set of bearings through the body and the 90 degree gear box. I was able to lower mine to about 6 inch pounds by cleaning the grease from the U joints, bearings and then replacing it with light weight synthetic grease. I replace the 90 degree box with the one from the transit bus. The original 4104 box had bronze bushings. The box from the transit bus had tapered roller bearings, even with that I still had to reduce the pre-load on the bearings to a point that was border line, increase the clearance between the bevel gears and use the lightest synthetic motor oil I could find. After all this I had the return to center at an acceptable level, not perfect, but it was OK.
Now I still had the excess free play. As far as I know none of the axle mount Sheppard gears have a free play adjustment. While most every Ross gear has the adjustment. The problem was the only axle mount gear Ross every produced was used on the MCI A2 and that gear is no longer made and I could not find a used one anywhere.
I went to a truck scrap yard and found two Ross gears, one with a sector shaft long enough to get below the axle and another gear with correct housing orientation that would allow me to build an adapter plate for the axle mount. I used parts from both gears to build the one I needed. I used a pillar block bearing to stabilize the bottom of the sector shaft because there was quite a bit of unsupported length between the gear and the pitman arm. After ever thing was connected and adjusted I went for test ride and I was pleased, it drove about 95% as good as the MCI 102A3.
One other thing, centering up the steering gear. Back out the internal stops or reliefs in the gear, rotate the sterring wheel full left to right counting the turns. Then bring it back to the exact center. Lock the steering wheel in that position (ignore the position of the wheel for now) and adjust the drag link to make the coach drive straight with the steering wheel in that position, then remove the steering wheel and position it to the center if necessary. That will keep the steering gear centered. Most steering gears are designed to be a little tighter in the center position. It's not uncommon for the steering wheel to be centered up for driving using the drag link adjustment. That is not the correct procedure and now you know why. I pass this information along hoping it will benefit someone.
I used the upper front axle radius rod from a 4106. It's adjustable and will work on a 4104. It will allow some caster adjustment or most truck alignment shops can place a wedge shim under the axle to change caster. Uncle Ned's bus does drive great with the Sheppard gear. So not all Sheppard gears are bad, just the ones I saw.
I know some of you have more alignment experience than me. If any of this is incorrect please correct me