I would check the vents for the carb. If they are not vented the fuel will not flow. If you have a clear filter on it, you should be able to see if it's clogged. You say when you disconnect the fuel line the fuel flows. Well, are you disconnecting it from the inlet or the outlet side of the filter. If you disconnect it from the outlet side and the fuel fills the filter back up, then you have a vacuum lock problem and most likely plugged vent lines (since you said the fuel won't flow when you take the fuel filler cap off, I'm assuming the vent for the tank is working properly, but you should check that also). If the fuel only flows when you unplug the inlet side of the filter, and not the outlet side, then you have a faulty filter that is somehow getting plugged up. Just replace it with a good high-flowing clear with brass filter element and you should be fine. I would recommend if you are going to fun external filters that you run two filters....one on each line, and put the t before the filters.
Also, if you don't have to run the bike with the rusty tank, then don't. It's not a good idea and it can create lots of problems for you. Get it cleaned or get a new/used tank.
Someone help me with this, but I also seem to remember reading something about the fuel not flowing without vacuum from the carb being connected to the petcock. Is this right, or am I just imagining it. Perhaps your vacuum line to the petcock is collapsing in the heat, but not so much at night in the cool.
Good luck and keep us updated.