engine light on 2000 chrysler cirus

Check engine light came on.Bought new gas cap and put it in shop.They checked code .It came up vapor leak.He hooked up tester and found rusted filler tube for gas tank.Patched it and checked system agian for leaks .Thought it was fixed so Ibrouhgt it home ,next day light came on.Put back in shop .He took tank down checked everything and other test hooked up to tester and cant find anything leaking.Picked it up this time wife drove to work 2days before light came on.Car is a 2000 cirus 4cy auto air.Runs great has 150000 miles still has original exhaust system .I am going to bring it back to shop THURS. have any ideas . This time code came up for plastic valve on top of gas tank.Replaced that but light came after one day of driving. Still up for sugestions.ThankYou. Finally found leak at fill tube and tank.
 

These types of leaks are difficult to find. It sounds like you are being resonabable and the shop is performing their do diligents to find the leak. If they are using some type of smoke machine to find the leak (which it sounds like they may be) then they should find it given time.