I had a customer call me asking about an air gap that leaks. He was trying to diagnose the problem and was haphazardly checking under the sink every now-and-then. So he called to learn a little about his dilemma.
The air gap sits on top of your counter-top. Underneath the counter is where the magic happens. It has two outlets on it…one is smaller than the other.
The smaller outlet is connected to the dishwasher drain hose and the larger outlet is connected to the garbage disposal inlet (if you don’t have a disposal, it will be connected to a nipple on the tailpiece. So the answer to the question is that if the air gap will leak, it will leak when the dishwasher goes into its drain cycle. This is the only time there will be water passing through the hose and the air gap.
If you find the air gap leaking under the sink, check the connections where the hoses attach to the air gap and the connection where the air gap drain hose connects to the garbage disposal. These are usually connected by hose clamps, which are very reliable, but they may need to be tightened. Another common culprit is the garbage disposal itself, so be detailed and check that too.