Why doesn't fuel flow from the tank?

[fiddler]

I have a titled 1974 Chevy c10 stepside.  I say titled because it has so many replacement junkyard parts on it.  I think the engine is from a 1976.  Anyway, it had an intermittent fuel starvation problem so rather than fool with it I just pointed the parts cannon at it and replaced all of the filters and the fuel pump.  In the process I prepared to plug the input to the fuel pump when I removed the hose.  But no fuel came out.  I put some air pressure on the top of the tank, and fuel comes out of the fuel line, but it doesn't free flow.  Appears not to be an issue, but can anybody tell me why that is?  Thinking maybe some kind of fuel line routing or special valve?  Oh the intermittent appears to be solved, but... it's an intermittent so it's going to take a few days to call it fixed....

[zucchi]

There's usually a screen on the opening end of the pickup tube inside the tank. Remove the sending unit/pickup tube assembly and examine.