MY 1990 F-150 HAS TWO GAS TANKS. FOR THE PAST WEEK THE GAS HAS BEEN GOING BACK AND FORTH BETWEEN THE TWO TANKS. I KNOW THIS BECAUSE I CAN FILL THE FRONT TANK AND PUT 1/4 TANK OF GAS IN THE REAR TANK. THEN AN HOUR LATER THE REAR TANK IS HOLDING THE GAS FROM THE FRONT TANK. THE BIG PROBLEM HAPPENED TODAY WHEN I FILLED THE REAR TANK AND THEN PUT $5 WORTH IN THE FRONT TANK. I HAD A 70 MILE DRIVE HOME. WITHIN THE FIRST 20 MILES THE REAR TANK WHICH I HAD FILLED, WAS READING 1/4 FULL! SO I SWITCHED TO THE FRONT TANK WHICH WAS EMPTY EXCEPT FOR THE $5 WORTH OF GAS I HAD PUT IN. HOWEVER, THE FRONT TANK WAS NOW READING FULL! OBVIOUSLY THE GAS HAD GONE SOMEHOW FROM THE REAR TANK AND UP TO THE FRONT TANK. BY THE TIME I GOT ABOUT HALF WAY HOME, THE CAB OF THE TRUCK WAS FULL OF FUMES. I STOPPED AND GOT OUT BECAUSE I DEVELOPED A TERRIBLE HEADACHE. THERE WAS GAS POURING FROM THE UNDERSIDE OF THE TRUCK. UPON CLOSER INSPECTION, I REALIZED THE GAS WAS SPILLING OUT FROM THE REAR TANK AT THE AREA WHERE THE GAS GOES IN AT. WHEN I CHECKED THE GAUGES THE REAR TANK WAS NOW FULL (REMEMBER 30 MINUTES EARLIER IT WAS ONLY 1/4 FULL AND THE FRONT TANK WAS READING FULL),. THE TANK WAS OBVIOUSLY OVER FULL BECAUSE IT WAS SPILLING OUT FROM THE AREA WHERE THE HOSE GOES IN TO FILL THE TANK.*AK