1968 DODGE DART Tires Problems

ON 12/6/06 I PURCHASED A SEARS TIRE 185 80S13 FOR MY 68 DODGE DART AND GOT CHARGED FOR A NEW VALVE. THIS TIRE WAS PLACED IN THE TRUNK AS SPARE. ON 4/10/07 I PURCHASED ANOTHER 3 OF THE SAME TIRES, INCLUDING VALVES, AT SEARS AT 95668 MILES ODOMETER READING AND HAD THE TIRES, INCLUDING THE EARLIER PURCHASED SPARE, MOUNTED ON MY CAR. ON 11/14/07 WHILE DRIVING ON THE FREEWAY, MY LEFT FRONT TIRE BLEW OUT AND I DISCOVERED THAT THE VALVE WAS CRACKED AT THE BASE. THE SAME DAY I PAID SEARS TO REPLACE THE VALVE. THE ODOMETER READING WAS 96382. ON 7/4/09 I DISCOVERED THAT THE RIGHT FRONT TIRE LOST ALL AIR WHILE SITTING IN THE GARAGE WITH AN ODOMETER READING OF 97561. AN INSPECTION OF THE VALVE REVEALED THAT IT TOO WAS CRACKED. NOTE THAT I DRIVE THIS CARE VERY LITTLE AND MOSTLY IN THE CITY. THE VALVE...