P0442 - Evap syst (small leak)

P0455 - Evap syst (lrg leak)

These can be tricky to track down without the right tools. Those hard plastic lines can become brittle over time & crack. You didn't mention, but if any fuel tank or pump wokr has been done recently, you want to check the line connections to the pump or pump sending unit o-ring. If no leaks from lines, filler tube, gas cap, etc it could be one of the solenoids not sealing properly when it is supposed to. GM's are notorious for canister vent valves not sealing or failing all together.