You could, as I say, have blockage as your problem. That is why you use a process of eliminate/isolate. This is useful if you don't have a lot of testing equipment. It is sometimes much faster and less chance for error or misinterpretation.
With the gas can you are eliminating the tank/blockage, the tank pick up, the screen, petcock, fittings, filters, cap vent and hoses and isolating one component, the pump. This is a quick way to focus on what area of the system has a problem. If pump does not squirt, it's the pump. If pump begins to squirt out the outlet, you hook up outlet to carb and run the motor from the can. (BTW: Set the can on the ground). If motor runs, it's a line/tank problem. If it does not run, probably a carb problem. With just a gas can and a hose barb fitting, you analyze the entire problem. No vacuum testing, taking tank off, replacing pump, lines, filters, forcing gas into lines, rebuilding carb and so forth. This isolates and eliminates a good portion of the fuel supply system. If it turn out to be lines/tank, then you isolate and eliminate that area where possible. More on that as first results are known.
I don't know too much about the sight glass fuel pump. I don't use them or like the idea. They can possibly leak gas, spill gas or suck air and are not necessary, except for looks. It is slightly possible it is doing something to cause this. They say it's a filter, a sediment bowl, a water separator and assurance. I say, that's a bunch of "horse and buggy". Filter your gas BEFORE the pump, a completely sealed pump!
("nothing at all came out")
I had the same concern as Carl but decided you meant no gunk came out. Not that no gas came out. Only small particles will come out, larger may float around and maybe block when flow/suction starts, as Carl says.
