assuming there is no crack from you drilling the hole, you might want to remove the bulkhead and make sure its clean from any dirt, especially the rubber O ring. I would suggest to use the bulkhead rubber O ring because they're wider then the O ring pictured above... and specifically designed for the bulkheads. If there is some dirt or deterioration on the O ring, clean or replace it. When putting it back on, make sure its centered on the hole. Hand tighten as hard as you can without tools and try again.