After several recent fuel-pump failures I find myself with a stack of bad main pumps.

What fails on them? Has anyone repaired/ rebuilt one?

I disassembled one yesterday. It wouldn't turn (from 12V or by had when I got it partially apart. I couldn't see what had failed. It might have been that the plastic impeller broke/ slipped; or that the rear bearing froze.

I thought I might be able to make 1-2 good pumps out of 5-6 bad ones. Better than just throwing them away.

I also have several bad lift pumps but I doubt it's worth digging into these as they're only around $35 new. But a $135 main seems worth some effort.
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240s: 2 drivers and some parts cars

Might have to order new parts - I'm sure it can be done, they reman pumps all time. In fact, reman pumps can often be had on eBay for like $40 so...would it be worth your trouble?

I'm guessing - very much so - that what dies on pumps is the winding after it draws too much current from pushing too hard. Not an easy thing to fix, I've seen my father rewind old magnetos and such.
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'93 244: 'A' cam 4 deg. advanced, 25/22 sways, custom heim endlinks, poly bushings, and a lot more styling customization than I care to recount.

Yes, I remember folks doing that years ago too. If re-winding the motor is required, no, it's not worth it. I thought it might just be brushes, an impeller, some small bits.

There's a pattern in my recent failures: 3 pumps were pulled from working cars and left to sit on a shelf ~ 1 year. Does something dry out when they sit?

I'll pull a couple of more apart this month and see what I find.
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240s: 2 drivers and some parts cars