I had my ball valve about 3/4 of the way up and had the same issue. There has been much discussion on this subject in the mother thread, but when I put mine at the bottom the issue went away. My advise is to screw it straight into the nipple in the drum.
Weber Imperial Sequoia (restoration in progress)
Weber Platinum (The Green Lantern)
Weber OTS 22.5
UDS (with more to come)
Old Smokey ES (A gift, don't buy one)
Humble member of the Society for the Preservation of Authentic Royal Magical Rare Kaskaskian Peppers
Honor bound to protect, prepare, and serve