my sons 89 D350 cummins has lost the power steering fluid twice the last 2 days.It has new lines and there is no evidence of a leak. Could it go by the front seal and end up in the engine oil? Birdftracker

or vaccum pump? Birdtracker

Is it possible that there's (or was) air in the system, and it's just working itself out?

i was thinking about the air working itself out as well, as i have had a few instances similar to that over the years. in my circumstances, the fluid didn't completely disappear, but turned up very low in the reservoir several times until it stabilized at the recommended level, then stayed there.
i determined it had to be an air issue, as no leaks could be found [just as the OP] and the steering operated as it should.
as i know nothing about the diesel engine, how would it be possible for the power steering fluid to leak into the engine ? is the pump driven by an engine shaft ?
curious to know.

On the Cummins trucks, the vacuum pump is driven by the timing gears. The power steering pump is driven by the vacuum pump.

I'm pretty sure there's an air gap between them so if the power steering pump leaks it's external.

we have filled it up 3 times now. It seems to be going somewhere even sitting over night. Think I will throw a piece of cardboard under it over night. Birdtracker

If you have filled it three times, it is diffidently leaking somewhere. My guess is a PS line. The card board trick my find it.

Its an old truck my son and I are keeping alive. Its old but suprising its dry. That's what doesn't make sense. And of course cause its an 89 and non intercooled its a little more complex to get the pump off.Rain and weather and eating will probably slow us down some! Birdtracker

How much are you adding each time?

more than a pint but less then a quart. 2 1/2 bottles. Birdtracker

My 2001 Dakota has been doing that same thing for a while. Months ago, no issue at all. Now, I fill the PS reservoir and by morning, desert dry.

well weather turned to crap here in Ohio. My son took the vac pump power steering pump assembly off. I took it to work and pulled the gear off and swapped off the pump. I looked in the vaccum hoses and did not see anything. If its anywhere I am guessing it went in the booster. I think when it gets nice one day I might take a long plastic wire tie and put it down to the bottom of the booster and see if its there. Birdtracker