I can't give first-hand information on a full overhaul of the AC's steering, since my Bishop Cam steering-box was in good order. Despite the terrible things owners of pre-war Bishop steering-boxes say, this post-war version seems to have addressed most of those old design issues. Of course, it always helps if one puts suitable lubricant in it!
The steering-box features a helical cam. Although it resembles a conventional worm-gear, its profile is more complex. That is because it engages with a peg that moves in a curved path (mounted on a rocker-arm), so that it swings off the cam's axis. The cam profile keeps free-play to a minimum, although the design does produce a gradual increase in play as the steering moves away from the centre towards either lock. The peg is mounted in both ball and roller bearings, which identifies this as the "high-efficiency" version of this box. It doesn't provide as much damping for road shocks as earlier designs of the Bishop Cam. The high-efficiency box also has an adjusting screw in the top cover.
Wear in the cam and peg are taken up with the adjusting screw on the top cover. If the steering box is still on the car, then both front wheels need to be off the ground so that you can feel any drag when turning the steering wheel. Slacken the lock-nut, and turn the screw clockwise until you can feel a hint of drag as the steering passes the straight ahead position. There should be no free-play at this straight ahead point, but a little play towards each lock. That is because most wear takes place near the centre of travel and allowance is provided so that it doesn't bind at each lock after adjustments. Don't forget to tighten up the lock-nut.
Note that the top cover has a paper gasket under it. If you need to renew this gasket, then be sure that a suitable thin paper gasket material is used, so that the adjustment isn't compromised (as might happen if a gasket gradually compressed over time).
The cam is mounted between a pair of ball-thrust bearings, and metal shims under the end plate permit adjustment. There should be no end play at all.
The AC handbook recommends Kamoil for the steering box, with EP90 gear oil as an alternative if the former oil is not available. When I ran my AC in the 1980s, I tried EP90, but this leaked out passed the cork seal on the sector shaft. I substituted SAE 140 heavy gear oil which still leaked slightly, but stayed in long enough to be useful!
Don't be tempted to introduce grease to solve a leak, unless it's a semi-fluid grease made for this application. Talking of which, Penrite produce a steering box lubricant which they describe as a semi-fluid lithium based grease. Its base oil is a similar viscosity to the EP90 oil I once tried, but less likely to leak out.
Millers also do a steering box lubricant, at the time of writing (2017).
If changing the type of lubricant, then one needs to remove as much of the old oil as possible. Removing the end plate might drain most of it, and taking off the top cover to clean out what is left.
Firstly, the wiring for the horn/indicators has to be pulled out. Prepare to catch some oil leakage as you undo the brass nut on the end of the steering box where the wiring emerges. Remove the olive. Then slacken the grub screws on the steering wheel boss, which will release the horn-push assembly. You can then pull it out along with the tubes containing the wires. A wire circlip has to be prised off to allow the steering wheel to pull off the splined shaft.
Disconnect the draglink from the drop-arm using a suitable puller. The AC manual says that you should remove the drop-arm from the sector shaft, using a puller and then unbolt the clamp holding the sector shaft. That looks like an awful lot of bother when you can simply remove the large bolt that secures the box to the chassis (and acts as a pivot when adjusting the steering wheel height).
Unbolt the handbrake cable from the column. Remove the rubber around the hole in the bulkhead. I would recommend sticking some padding around the inside of the bulkhead hole, to avoid scratching the steering column. Remove the bracket at the top of the column, then the main bolt supporting the steering-box. Lift the box up high and pull the column out.
This is the largest bolt on the car: 5/8BSF x 3.5", high tensile grade D (equivalent to the later grade R). I couldn't find new bolts of this size, so I crack tested mine. The split-pin hole had been drilled off centre on mine, with a notch filed into the slotted nut.
The mounting bracket for the column has a rubber bush. If this is a slack fit, the precision of the steering will be poor. Turning the steering wheel to the right, makes the column shift to the left (also stressing the steering-box mounting bolt) unless it is firmly held in place. I cut out a new bush from a thick sheet of natural rubber and obtained a firm fit.
Column mounting bracket with a new rubber bush (old bush shown to the right).
I haven't overhauled a steering-box, but I'll give some pointers from what I've read in period textbooks. On my own steering-box I noted a hint of tilting of the drop-arm, suggesting slight wear in the bushes for the sector-arm. New bushes can be made, installed and reamed. A new cork seal will be required below these bushes. The ball-thrust bearings for the cam might also need renewal. If the cam and/or peg themselves are too worn to re-use, then you have a challenge to obtain replacements, or to have new ones made. The profile of the cam needs to match the complexity of the original. Attaching the cam to the column is - apparently - a specialist task. The shims are then used to adjust the end-play of the column/cam.
This should be straight-forward. The main mounting bolt is inserted through the chassis, then slide on the big washer and then the distance piece followed by the steering-box. Make sure that the column/box can pivot around the main mounting bolt. Apply a bit of grease, and take care not to tighten the slotted nut too much before fitting a new split-pin.
To get the wiring back in, slide the lower tube to the end of the wires so that they can be guided through easily. Once you've pulled the wires fully through, slide the lower tube back up the column and join the 2 tubes together. They should be able to slide telescopically. Then slide the tubes back down, fit the olive over the bottom end, and fit the coupling nut. That will hold the horn/indicator switch and stop it rotating with the steering wheel. Then tighten the grub screws around the steering wheel boss. These grip the self-cancelling mechanism for the indicator switch. The steering box oil can then be topped up.