Update on an old Burb.

[DmaxMaverick]

It isn't making sense. Seeing is believing, but I'm not seeing. Air has to come from the pump. If the pump is pushing fluid, everything is under pressure, including the return lines when they're returning. The HB unit can't "pull in" air. It can only push excess flow out the return, or to the PS gearbox. If it's foaming, it's coming from the pump, and it takes very little air to make a lot of foam. If there's air in the system, it will exit the HB unit return first. The PS gearbox return is much lower, and it will take much more volume before any air will show there. The pump is pumping air, from somewhere. Most likely a defective pump, with no evidence of a leak, anywhere.

[DickWells]

Okay. Well, then, why doesn't it push foam out the PS return? This is why I posted the question about further testing and possibilities. IIRC (You'd think I'd know,. I was just out there this afternoon!), there's just one pressure line from the pump, which goes to the HB, then the steering gear. So, I think my next step is to lift the car, again, and try the steering, lock to lock a few times. We only tested it , running, today, with wheels on the ground and turning back and forth enough to hear the groaning. I'm inclined to think that I wasn't thorough enough.
BTW, I already have a rebuilt pump, pump-to-HB hose, filter and fluid, so, I'm set to tackle that end, if I do prove that the pump is bad.
Thanks for all your good input.

[DickWells]

Update. Got the new (OEM rebuilt) PS pump in. During the process, while using the tank and pully off the year-old pump, I discovered that the magnet was dislocated from the flat side of the casting and was covering the end of the supply through-hole! Not completely, as the surface there is concave. So, I checked my new/rebuilt pump and, sure enough, there's the magnet, covering the hole! Now, having 3 magnets to choose from, I found that the old, original pump, had an obviously stronger magnet, which I cleaned and installed on the proper flat in the pump. Installed, flushed and topped up the system, drove a couple of miles, with nary a moan. Topped it up, took a longer drive and topped it up, again. Have ~40 miles on it, and it looks like Maverick hit it right on the head. I don't know for sure, having never been through the issue, before, but it seems logical to me, that the magnet, nearly blocking the fluid supply on one side, would create a lot of cavitation in the pump, at the very least. Anyway, this pump required no fiddling with purging at all, so far. Unfortunately, there are now other issues, with getting the 30+ year-old 6.5 to keep running (See "Where do I start."), elswhere.