Hopefully it isn't this, but I wonder if the metal pull rod has come out of the valve blade and so the blade just remains in place (closed) when you pull the rod.
I haven't experienced this, but it is something I have always been concerned about.
If that is the problem, then unless anyone has any good suggestions, I would guess it could mean having the black tank pumped out via the toilet and then replacing the valve assembly. Maybe best left to an RV dealer!
Sure hope I'm wrong, and maybe others have different suggestions as to what the problem might be, and/or if there are easier ways to deal with this. I'd like to know myself for possible future use.
Brian & Connie Mitchell
2005 Classic 30'
Hensley Arrow / Centramatics
2008 GMC Sierra SLT 2500HD,4x4,Crew Cab, Diesel, Leer cap.