I have a fairly new Ryobi string trimmer. When it's stone cold, it
starts fairly well, and starts good at operating temperature if it
doesn't sit around for more than about 1 minute.

When the trimmer sits for more than a minute or two "hot," it won't
start for love nor money. The problem is that there is no gas in the
line! Gas won't stay in the line. I choke it and pull to get gas to the
carb, but by the time I unchoke and pull again to start it, the gas has
all boiled back up into the tank. I use the primer bulb to fill the
fuel line, and as soon as I stop pumping, the gas bubbles back up into
the tank!

What is going on? What can I do about it? I suspect an air leak, but I
can't find one. All the fuel lines look solid and intact.