I found mounting the tank first before screwing the top and chimney on cured thatThe o-ring on the base would stop it leaking from the sides of the tank.
One of my Pioneers' is a P.I.T.A when screwing the base back on.
If I don't get it right there's a 1mm gap at one side between the base & the bottom of the tank.
If your tank is sitting like this then maybe the chimney isn't sitting flush with the deck too? Just a thought.