I just bought 2 - 5gal corny kegs, and it’s the kind with the top with a latch. You pull a bar across and it pulls the lid up and is suppose to be tight, but I noticed when I was cleaning them I put them upside down and they are leaking every where. Maybe I bought ones that need to be reconditioned?
Just closing them may not seal them completely. Hit them with a blast of CO2 to seal them then turn upside down to check. Sometimes reseating the lid also is enough.
+1
And some keg lube.
Sometimes you have to hold the keg in the air by the bailing latch (or whatever its called) to apply enough manual pressure to allow the CO2 blast (20 PSI usually does the trick) to pressure-seal the lid
You also may need to replace the o-rings.
I’ve had some success when all else fails by bending the two legs (slightly!) inwards/together. Seems to make it a little tighter. Usually though, just reseating works. All these used kegs have their own little quirks and they might take a little getting used to. 