MFL wrote:
If you have the screen vent, next to gravity fill, it is unlikely that there is any other vent. I would suspect loose/leaking fill hose where attached to the tank, or possible detached breather hose. If breather is working right, you should feel air coming out screen when filling.
You may need to cut a three sided window in under cover, where hoses attach to the tank.
Jerry
I was hoping to avoid cutting into the under cover. But I will if that's the best. Any chance from getting there under a cabinet of some kind?