When that exact same thing kept happening to my 62 Allis D-12, I finally took the fuel line off to the carb to drain the tank. Started out as a stream, slowed to a trickle, then to a drip. Hmm... full tank but no fuel coming out. Drained and removed tank. Looked inside and saw rust patches and loose rust chips. The chips were to big to get through the petcock to the sediment bowl, but plugged the petcock up cause its at lowest point in tank. There was no screen on tank side of petcock so what I did after cleaning the gunk out of the petcock was put a 1" long piece of 1/4" copper tube into the petcock and put a stainless steel screen bonnet on it. Put petcock back in, refilled tank and problem tractor ran fine. This was just a jury rig though.
The only way to "cure" the problem is to get the rust out of the tank then coat the inside with liner.
Hope this helps.