Well, I have a few minutes to catch my breath here, so I will try to answer the questions that have come up on this thread.
First off, thanks, Jim, for posting this. I was caught completely off-guard by this, and your post here may save someone else's bacon. It was only through good fortune and/or timing that three knowledgeable people happened to read my blog post that night, and sent me emails before we left in the morning.
Brian Diehl wrote: "I wonder why he is using ATF?"
The answer is: Because that was what was in the system when I got the bus. I don't have any documentation or specs on the system, so I have no idea what the allowed or required fluids are -- I just continued using what was already there.
My system, by the way, is bastardized -- the steering box is original equipment on the bus, and is probably a Mercedes part. The bus was built in Pilsting, Germany, and the hubs and axles are all Mercedes. None of the front-end steering gear has ever been replaced, so it is 22 years old.
The engine, on the other hand, was replaced here in the US when the bus was only a couple years old. It likely had either a Mercedes or Deutz engine in it when it was built, and Neoplan USA swapped it out for a brand new 8V92TA Detroit. The gearbox was replaced at the same time -- again, OEM was likely either ZF or Mercedes, and was almost certainly manual, and it was replaced with an Allison HTB748. The pumpkin, however, is OEM Mercedes. The mate-up looks to have been a torch-and-weld job. In any case, the power steering pump must have been changed with the engine, from a (probably) belt-driven one off the Deutz or Mercedes to a gear-driven unit on the Detroit. I'm almost certain the pumps are not interchangeable between these applications.
"makemineatwostroke" wrote about the differences between Eaton and Vickers pumps. I don't know which I have -- without getting the bus on a lift or over a pit, it's nearly impossible to get to, so I can't look. I am guessing Vickers, because those are, apparently, the ones with the tendency to drain back into the engine sump when the shaft seal goes. At least, according to the gentlemen who saved my butt by writing in to warn me about this. And while the Vickers may be fine on motor oil, it's not clear that the steering box will be.
Lee Bradley wrote that I had a hydraulic hose rupture on my steering system a while back, and this is true. We never lost enough fluid, though, to even drop below the sight glass. That said, replacing the hose introduced a large air bubble into the system, which took quite a while to work all the way out. The steering would "quit" on me at random times, usually while trying to parallel park. I suspect the air in the system contributed to the failure of the 19-year-old pump seal.
TomC wrote that his Sheppard steering uses 15W-40. And I will tell you that what I did, to safely continue the rest of the way (700 miles) to Portland from central California (disaster victims can't wait), was to simply start adding 15W-40 to the steering reservoir instead of ATF. I also had the engine oil changed at the very next express lube shop we passed, also to 15W-40 (none of these places stocks 40-weight any longer, and the multi-grade was certainly better than the 30% mix of ATF I had in the crankcase, and fine for the 700 miles it was going to be in there).
After running the 700 miles to Portland this way, I can report that my power steering is NOT designed to run on 15W-40. It sounds like a rutting moose when the engine is first started, and much worse when the weather is cold (it's 35 here tonight -- and we were in Palm Springs, where it was 80, when they called us). It also shudders a bit at low speed maneuvering, especially when cold. That said, I'm pretty sure the 15W-40 is not doing any actual damage. I'll switch back to ATF, or maybe try straight 30-weight, when I get the pump repaired. By the way, I have no idea how much fluid my steering holds in total, but the reservoir alone is well over two gallons. When the steering "quits" due to low fluid, it takes around two and a half gallons to fill the reservoir. After starting the engine and making a couple left/right wheel cranks, there will be room for another half gallon. So maybe twice as big as Lee's "six quart" system.
Only time will tell if we thinned the oil enough to do engine damage. I will need to flush the oil system, refill with 40-weight, and run a few thousand miles before I can pull a meaningful oil sample. And, yes, for the record, we have less than 10,000 miles on a complete in-frame rebuild right now. I have my fingers crossed that we did only minimal damage to the bearings, rings, and liners. On the bright side, the oil passages are very, very clean now.