I have a 73 C20 that I have used since it was new. It is a great daily driver that has taken me across the US and back. Since the early eighties it has developed a problem at the filling station. 9 out of 10 times the tank will only fill to the half way mark on the gauge then abruptly shut off at the nozzle, spilling gas all over the place. I live in Oregon so I can't baby the nozzle to get it full. Every now and then I get the nozzle when the attendant is busy and I can get a full tank. I know from what countless attendants have told me that this is very common for these older Chevys. Anyone know what the problem and fix is?