Tire Valve problems of the 2002 Toyota 4Runner

6/9/08 after leaving the car parked at a store for 20 minutes, the front right tire was totally flat. Had car towed to north shore firestone and tires inc . . They said the valve failed. On the tire. . . I had the shop replace the valve and rebalance the tire. . . . . . . . . 7/16/2008 I was about to go to work and found the front left tire. Missing half of the air. . . I took out my cordless air pump ,,attached it to the valve and felt air coming out of the valve, at the stem. . . Had it towed to my mechanic ,huntington diagnostic in huntington and had the front and back 2 valves replaced . . I also had the service station keep the 3 valves for me to pick up when the work was completed. . . . . . . . . The tires were originally replaced along with the 4 valves August /2006 the car had 55k from nys discount tire 1765 NY Ave huntington station NY 11746 631-423-8473 ronnie/owner at the time of the 2 flats I had 75k and 77k on the car. . . . . Ningbo yinzhou haixin tire valve CO ltd----makes the valves.