Hello. If your shutting all your intakes and exhaust, and its not snuffing the charcoal, then it's pulling air in somewhere. I've got 3 intakes that are 2" up from bottom of barrel. They use 3/4" close nipples, 2 w/pipe caps and 1 ball valve. As far as exhaust goes, I'm using a weber lid. I can close all intakes and the smoke coming from the exhaust immediately starts dying. Then once exhaust is shut it goes out reasonably quick.
I've been amazed at how well mine will hold a steady temp. That's with briquettes or lump. These things are awesome!
Btw, you got the liner out, right?
UDS, mini wsm, 22.5" weber kettle