Sorry for the delay in posting. Real Life interfered in my playtime.
Removing the 12V PSU required opening the AC feed box in the power supply sled. This was clean as a whistle inside, since it's a sealed enclosure.
You can clearly see where the protective cover was.
The 12V supply looked clean as well, with no visible rust except for the terminal screw heads.
The insides looked good as well - This is promising!
So we have one of two supplies needed to run the machine. I pulled the 5V PSU from the other Lambda. It was rustier on the outside, so I had assumed it to be the worst of the two, but once it was opened up, it turned out to be in far better shape.
Only dust! Looks like we have a pair!
Both power supplies were cleaned out as best as I could and reassembled. The cleaner innards from second 5V PSU was replaced into the cleaner case from the first 5V PSU. After that, both were tested and determined to be within acceptable parameters at no load.
So now we need a PDU to feed them with. I was expecting this to be a wash as the breakers from neither would stay closed even with no input, and the second one had visibly corroded connectors. So I pulled both and opened them both. As expected, it was corroded inside as well. The first one was clean though!
So now why aren't the breakers working? Nothing looks bad outside...
I figured they both must have the same issue, so I pulled the breaker assembly from the rustier PDU to use as a practice run in case I wrecked the assembly getting it open. They were held together by four pins which were trivial to knock out. The insides were found to be contaminated with this black guck that had congealed on the moving parts, making them not turn as a group and trip the breaker immediately. I found that it scraped off showing clean metal underneath, so it must have been rust or mold or old lubricant or something. A minute or two with a Dremel cleaned that all out, and the breaker assembly operated as expected!
Since I had knocked out the pins, I reassembled the unit with electrical tape. I made sure to cut the tape away from any holes.
The unit was reinstalled in the PDU and the PDU was checked with no issues. The PDU and PSU sled were then reassembled and reinstalled in the rack.
Next order of business is to power the backplanes and see if there are any surprises there!