06 z71 5.3 throwing an evap major leak code..after clearing the light it comes on within 10 mins, the oring on the valve on the front of the intake manifold is good, No obvious line leaks, runs cherry, I suspect the gas cap, evap line or canister..really never had an evap system problem on the new chevys, Anyone know where to start, valve failure, busted lines, Any help would be appreciated thanks Tony