I'd take the tank off, turn it upside down, shoot some air back through the petcock and see if you can clear it. If you try to take the petcock off the tank, either use an impact or snap the ratchet quickly when you start to loosen those 8mm bolts. They're screwed into nutserts in the plastic tank, and if you don't snap them loose oftentimes the nutserts will start to spin in the plastic and then you have a whole nother issue to deal with.
If it were me, I'd flip it over, put the petcock on "ON" and then put a shot of compressed air up through there with my compressor. If that opens it up, flush out the tank and put it back on.
If it were me, I'd flip it over, put the petcock on "ON" and then put a shot of compressed air up through there with my compressor. If that opens it up, flush out the tank and put it back on.