John
Since you emptied the carbs, the filter, and the line to the LR tank, it will take a while to fill the carbs again even if the pump check valves are good, I'm guessing 50 strokes?
The pumps suck fuel from the tank so you won't get any fuel flowing with the filter canister off because you have a huge vacuum leak!
I suggest installing a tee in the fuel line downstream of the filter so you can temporarily connect a handheld vacuum pump like a Mityvac to suck fuel from the tank into the filter. Or you could remove one of those nice brass knurled vent plugs from the top of the filter, drill it, and solder on a connection there.
Putting air pressure to the tank to push fuel to the filter may not be working now because there in a vent on the LR tank, that strange rhomboid shaped block sitting at the rear of the tank cover. It has small vent holes on the underside. I managed to get the tiny Allen screws out (I think I used heat) and remove the vent and found the air passage underneath completely blocked by sand and dust. When you applied air, maybe you blew a similar blockage out.
I don't think bad check valves in the fuel pumps will allow flow back into the tank because the tank outlet is it at top of the tank and the carbs can't drain and the fuel pumps are much lower so the fuel won't gravity flow to the tank.
Did you take the tank top cover off? The standpipe going down into the tank had rusted away about 6" from the bottom on ours. I soldered a new section on to get the pipe extended back down so it fitted inside the fine mesh screen at the bottom of the tank. The screen is accessed by removing the smaller cover on the underside of the tank. The bolts on the tank are all SAE, not BSF, in case you hadn't noticed!
Malcolm