Bendix brake valve question

[PuroCumminsPower]

I think I found my problem. A air brake shop told me to disconnect the hoses from brake valve that say "del" and see which one is pushing air back into brake valve. The one pushing air back controls brakes. When I put my thumb over hose pushing air out its as if I was pressing brakes, trailer as well. Even lights come on. What valve would that be? I'm going to follow that air line but I don't know the name of the valve. I'll post a pic of air line. It has black tape on it. I'll update as soon as I confirm its fixed.