At the risk of being redundant
 

I opened the BB on my 78 Schwinn a few minutes ago. The plastic sleeve is not there, and there was some moisture damage. If I can't locate the one I have/had, what can I use as a substitute?
Thanks in advance.

Lots of BB's don't have the plastic sleeve. I suppose you could form one out of a plastic beverage bottle and some tape or glue.

Correct, the sleeve was by no means universal, particularly in OEM bottom brackets. My '85 Bridgestone didn't come with one. Try any older bike shop and they may have a few still put away in a bottom drawer somewhere. If the bb on your Schwinn is damaged, a cartridge replacement will bring it's own water shield with it.

You could make one from a plastic bottle or aluminium can

The plastic sleeve came about when folks started cutting holes in bottom bracket shells. If your BB shell isn't cut out, you're fine without the sleeve.

Back when, I used a sleeve made from a cut up beer or soda can, or even a greased file card. If everything has a grease coating such a sleeve will do an OK job of sealing up the bottom bracket and keeping debris and splashed water out of the bearings. As a bonus, you can write (or for the beer can) emboss information on the sleeve with a ball point pen that might help identify the bicycle if stolen.

I dunno. Water can, and does, get into the bb shell by running down the seat tube or possibly through the water bottle cage boss holes so a completely enclosed bb shell can turn into a pretty good aquarium if you don't check it periodically.

With the traditional cup/cone BBs that commonly use these sleeves, there's more than adequate venting for minor amounts of water that might weep in through the gap between the spindle and cups. While the sleeve does keep whatever falls down the seat tube, including bits of excess braze and flux out of the bearings, it also traps water between the sleeve and the shell preventing it from venting through the cups. I stopped using sleeves when I found that it seemed to increase BB shell rust. I went back to my old practice of smearing a blob of grease on the bottom of the shell to trap whatever fell into it.

However, if anyone's looking for these sleeves, I have a bunch and will sell them fairly cheap. email me c/o the Chain-L site.

Who's to say how long it's been since this thing has been opened up? It still had plenty of grease, but it also had water mixed with grease, especially on the drive side, and in the middle, where moisture coming down the seat tube would land. I'll put it back together sans sleeve, and see what happens, maybe check it in 6 months. Thanks for the replies, suggestions, and general info.

redundant. ohhhhh, man, a major memory there.
when i was a kid (maybe junior high) a friend (phil), his sister,
my sister and i were standing around shooting the bull. at one point
phil told his sister she was redundant.
now, at LEAST 3 of us 4 hadn't a clue what redundant meant- and,
knowing phil, he didn't know, either. just something that he had heard sommers.
well, sara, his sister, took serious exception to being called something that sounded
bad and commenced to beating on him (she was 3 years younger). put a hurt on the
boy.
i have never had occasion to use the word, but won't forget it!