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I'm wondering if somehow one or the other created a sump vacuum that would air lock the pump. I don't think either would, but strange things sometimes happen.

Nope can not be an issue. The higher the vacuum you can pull on the sump the better.
You are working with a positive displacement pump that is drawing from the same reservoir as it is pumping back into.
The vacuum helps with ring seal and reduces what the pistons need to pump in the bottom end.
I would say a stuck relief valve and I am glad you look up in there. I guess you did say the sump was full of metal so you had to look.

Cheers, Charlie
If It ain't broke, Modify it!
87 Montero turbo Converted back in Spring1989
95 Montero SR 3.8 DOHC Only one?
93 Pajero 3 door 6G75 Mivec with paddle shifted 5 speed
Then a Gen2 SR with full coil independent suspension.