What causes evaporative emission system leak? (Very small leak)

there are many parts of the evaporative system that can cause leaks, examples Canister Purge Control Valve,Canister Purge Solenoid,Evaporative Emission Control Canister,Evaporative Emissions System hoses,Positive Crankcase Ventilation system. need to have system tested to find cause of leak or leaks.

Typically time wears on hoses, vents, EVAP canister may reach the end of it's life span. Or could be as simple as the treads on the gas cap seal being worn. Then you would just replace the gas cap.