i've never personally made an overflow, but i have done quite a bit of research for making one, as i intend too. On a lot of youtube video's i've checked out about making overflows similiar to the stickies they always make sure to specify that it is very important to have a really thin airline tube inside of the overflow to allow air to escape, or go in? I'm not entirely sure the purpose of these tubes, but whenever they pulled the tubin out to demonstrate the sound of slurping was tremendously louder. if you have some small tubing for like an air pump, maybe you should put it in your overflow from the capped valve to see if it helps?