Just now looked at your picture. Don't completely submerge the gate valve. Leave a little above the water line. That should help tremendously as the air can rise to the surface before it exits the skimmer. Build an acrylic stand or use PVC cement to "glue" PVC "legs" to the black box. I bet that makes your problem go away. Your skimmer will work a bit better as well since you'll be pumping against less head pressure.
Dave
Dave