Thanks all of you -
These were really old kegs. My stepfather quit brewing in the mid 1990’s and they’d been in a garage ever since. When I tried to tighten it down, the plastic cracked, so it’s not fixable at this point. I’ll order some replacement valves since I’ll likely need a few anyway to see if that fixes it (and if it fits) and if that doesn’t do it, I’ll just get new lids. Thanks!