I’ve always used an airlock when cold crashing, but I’m tired of it (i.e., the negative air pressure threatening to suck water into my beer). So, today I said the heck with it and put a cap on them instead. Actually, I only had one cap that fit, so for the other two I just covered the stopper with duct tape. Am I missing something?
The glass carboys are not a problem for negative air pressure when you pick them up, but when they get in that cold environment, you can see the liquid starting to move the wrong way. And those *&^#@ Better Bottles create negative air pressure every time you pick them up. Plastic buckets probably do too, although I haven’t used one in a long time, but I expect to use them more in the future. So anyway, I figure, if you let the fermentation finish completely, it’s no problem. I know, I’m boring, too many words, this is like a diary. Have many of you long ago replaced your airlocks with caps for cold crashing? And yeah, I know, I could use vodka and not worry about it. 
I don’t use caps because I don’t have a cap that fits and don’t see a need to find one. If you use a one-piece airlock and only fill it a bit, you won’t have any problem. Only air bubbles will enter… not ideal, but air will enter when you take the cap off as well.
Use the S type airlocks and fill them just enough to make a seal(for carboys, I’ve no experience with buckets or BBs). That way they’ll admit just enough to equalize but not suck it dry.
That makes sense guys, thanks.