I had a very similar issue.
The check valve in the line would not open.
I was at a campground when this happened.
I called the dealer I bought the trailer from and the service tech helped me.
He told me to open the main valve very slowly.
When opened quickly, the pressure can cause the little check valves to stick closed. (Don't really understand that one but it seems to be acacurate)
I was very tempted to just drill that valve out. I didn't. I bought a new hose.
Hope this helps.
2009 Flagstaff 8528 RKSS
2007 Toyota Tundra DC 5.7L V8
Pullrite Superglide 14K
Air Lift 5000
Prodigy Brake Controller
"I've got a mind like a steele.. a......a steele......um.....uh...... what's that thing called again?