I guess one of the valves is stuck open, perhaps clogged open. Without opening the valves, I'd recommend first filling it with water and letting it sit for a while. That might break down the gunk in there, and if you pump it, it could move the gunk out. If that doesn't work then I think you'll either have to open the valves or replace the valves.
Glad to hear that you fixed the water gun. If it was a clog then just be careful about the water you put in. Use only clean water to avoid this. It might be worth adding a mesh filter to the pump intake if the Flash Flood does not already have one, too. That way anything that would clog the gun is stopped before it causes trouble.