1977 CHEVY 1/2 TON 350 - AIR IN FUEL

I HAVE JUST FINISHED REBUILDING THE ENGINE AND GOT IT STARTED, BUT IT IS RUNNING A LITTLE ERRATIC AT IDLE. I NOTICED THAT THE NEW CLEAR FUEL FILTER WAS NOT EVEN 1/4 FULL OF GAS. I DID A FUEL VOLUME TEST AND IT SEEMED O.K., BUT THERE WAS A LOT OF AIR BEING PUMPED WITH THE FUEL. I HAVE A NEW LINE ALL THE WAY BACK TO THE TANK AND THE TANK HAS BEEN CLEANED AND THE SENDING UNIT CHECKED. I HAVE 10 GAL. OF GAS IN THE TANK AND THE FUEL PUMP IS NEW. AFTER RUNNING THERE IS VERY LITTLE PRESSURE AT THE CARB (STOCK RODCHESTER). I CAN'T FIGURE OUT WHERE THE AIR IS COMING FROM? OR IS THAT NOT A PROBLEM? I CAN REMEMBER CHECKING OTHER FUEL PUMPS YEARS AGO FOR VOLUME AND DON'T REMEMBER AIR IN THE FUEL - JUST A SOLID STREAM OF FUEL. NOTE: I PUT THE OLD FUEL PUMP BACK ON (IT WAS O.K. BEFORE REBUILD) AND IT DID THE SAME THING - AIR IN THE GAS AND BARELY FILLED THE FUEL FILTER. BEFORE THE REBUILD THE FUEL FILTER WAS ALWAYS AT LEAST 1/2 FULL. ANY HELP WOULD BE APPRECIATED. JIM