my check gas cap light comes on, followed by check engine light. why??

What is your question?

replaced gas cap 3 times. new purge valve, new vent valve, new fuel tank pressure sensor. code is "large EVAP leak" or P0455. garage replaced vent valve 2 wks ago. garage replaced fuel tank pressure sensor 3 days ago. check gas cap light came back on. what could it be and i really don't think i should have to pay this time, because after the fuel tank pressure sensor they said they didn't complete the drive cycle!